Sequence: smallest number with exactly n divisors

Sequence: smallest number with exactly n divisors
You are encouraged to solve this task according to the task description, using any language you may know.

Calculate the sequence where each term   a_n   is the smallest natural number that has exactly   n   divisors.

Task

Show here, on this page, at least the first  15  terms of the sequence.

Related tasks

• Sequence: smallest number greater than previous term with exactly n divisors
• Sequence: nth number with exactly n divisors‎‎

See also

• OEIS:A005179

11l

Translation of: Python

F divisors(n)
   V divs = [1]
   L(ii) 2 .< Int(n ^ 0.5) + 3
      I n % ii == 0
         divs.append(ii)
         divs.append(Int(n / ii))
   divs.append(n)
   R Array(Set(divs))

F sequence(max_n)
   V n = 0
   [Int] r
   L
      n++
      V ii = 0
      I n > max_n
         L.break
      L
         ii++
         I divisors(ii).len == n
            r.append(ii)
            L.break
   R r

L(item) sequence(15)
   print(item)

Output:

1
2
4
6
16
12
64
24
36
48
1024
60
4096
192
144

Action!

Calculations on a real Atari 8-bit computer take quite long time. It is recommended to use an emulator capable with increasing speed of Atari CPU.

CARD FUNC CountDivisors(CARD a)
  CARD i,count

  i=1 count=0
  WHILE i*i<=a
  DO
    IF a MOD i=0 THEN
      IF i=a/i THEN
        count==+1
      ELSE
        count==+2
      FI
    FI
    i==+1
  OD
RETURN (count)

PROC Main()
  DEFINE MAX="15"
  CARD a,count
  BYTE i
  CARD ARRAY seq(MAX)

  FOR i=0 TO MAX-1
  DO
    seq(i)=0
  OD

  i=0 a=1
  WHILE i<MAX
  DO
    count=CountDivisors(a)
    IF count<=MAX AND seq(count-1)=0 THEN
      seq(count-1)=a
      i==+1
    FI
    a==+1
  OD

  FOR i=0 TO MAX-1
  DO
    IF i>0 THEN
      Print(", ")
    FI
    PrintC(seq(i))
  OD
RETURN

Output:

Screenshot from Atari 8-bit computer

1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144

ALGOL 68

Translation of: C

BEGIN

    PROC count divisors = ( INT n )INT:
         BEGIN
            INT count := 0;
            FOR i WHILE i*i <= n DO
                IF n MOD i = 0 THEN
                    count +:= IF i = n OVER i THEN 1 ELSE 2 FI
                FI
            OD;
            count
         END # count divisors # ;
 
    INT max = 15;
    [ max ]INT seq;FOR i TO max DO seq[ i ] := 0 OD;
    INT found := 0;
    FOR i WHILE found < max DO
        IF INT divisors = count divisors( i );
           divisors <= max
        THEN
            # have an i with an appropriate number of divisors                 #
            IF seq[ divisors ] = 0 THEN
                # this is the first i with that many divisors                  #
                seq[ divisors ] := i;
                found          +:= 1
            FI
        FI
    OD;
    print( ( "The first ", whole( max, 0 ), " terms of the sequence are:", newline ) );
    FOR i TO max DO
        print( ( whole( seq( i ), 0 ), " " ) )
    OD;
    print( ( newline ) )
    
END

Output:

The first 15 terms of the sequence are:
1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144

APL

((0+.=⍳|⊢)¨∘(⍳2÷⍨2∘*)⍳⍳)15

Output:

1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144

Arturo

firstNumWithDivisors: function [n][
    i: 0
    while ø [
        if n = size factors i -> return i
        i: i+1
    ]
]

print map 1..15 => firstNumWithDivisors

Output:

1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144

AutoHotkey

Translation of: Go

max := 15
seq := [], n := 0
while (n < max)
	if ((k := countDivisors(A_Index)) <= max) && !seq[k]
		seq[k] := A_Index, n++
for i, v in seq
	res .= v ", "
MsgBox % "The first " . max . " terms of the sequence are:`n" Trim(res, ", ")
return

countDivisors(n){
	while (A_Index**2 <= n)
		if !Mod(n, A_Index)
			count += A_Index = n/A_Index ? 1 : 2
	return count
}

Outputs:

The first 15 terms of the sequence are:
1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144

AWK

# syntax: GAWK -f SEQUENCE_SMALLEST_NUMBER_WITH_EXACTLY_N_DIVISORS.AWK
# converted from Kotlin
BEGIN {
    limit = 15
    printf("first %d terms:",limit)
    i = 1
    n = 0
    while (n < limit) {
      k = count_divisors(i)
      if (k <= limit && seq[k-1]+0 == 0) {
        seq[k-1] = i
        n++
      }
      i++
    }
    for (i=0; i<limit; i++) {
      printf(" %d",seq[i])
    }
    printf("\n")
    exit(0)
}
function count_divisors(n,  count,i) {
    for (i=1; i*i<=n; i++) {
      if (n % i == 0) {
        count += (i == n / i) ? 1 : 2
      }
    }
    return(count)
}

Output:

first 15 terms: 1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144

BASIC

BASIC256

Translation of: Ring

print "the first 15 terms of the sequence are:"
for n = 1 to 15
	for m = 1 to 4100
		pnum = 0
		for p = 1 to 4100
			if m % p = 0 then pnum += 1
		next p
		if pnum = n then
			print m; " ";
			exit for
		end if
	next m
next n

Chipmunk Basic

Translation of: Ring

Works with: Chipmunk Basic version 3.6.4

Works with: QBasic

Works with: MSX BASIC

Works with: Run BASIC

100 print "the first 15 terms of the sequence are:"
110 for n = 1 to 15
120   for m = 1 to 4100
130     pnum = 0
140     for p = 1 to 4100
150       if m mod p = 0 then pnum = pnum+1
160     next p
170     if pnum = n then print m;" "; : goto 190
180   next m
190 next n
200 print "done..."
210 end

GW-BASIC

Works with: PC-BASIC version any

Works with: BASICA

The Chipmunk Basic solution works without any changes.

MSX Basic

Works with: MSX BASIC version any

The Chipmunk Basic solution works without any changes.

QBasic

Translation of: Ring

Works with: QBasic version 1.1

Works with: QuickBasic version 4.5

PRINT "the first 15 terms of the sequence are:"
FOR n = 1 to 15
    FOR m = 1 to 4100
        pnum = 0
        FOR p = 1 to 4100
            IF m MOD p = 0 then pnum = pnum + 1
        NEXT p
        IF pnum = n then
           PRINT m;
           EXIT FOR
        END IF
    NEXT m
NEXT n

Run BASIC

The Chipmunk Basic solution works without any changes.

True BASIC

Translation of: Ring

PRINT "the first 15 terms of the sequence are:"
FOR n = 1 to 15
    FOR m = 1 to 4100
        LET pnum = 0
        FOR p = 1 to 4100
            IF remainder(m, p) = 0 then LET pnum = pnum+1
        NEXT p
        IF pnum = n then
           PRINT m;
           EXIT FOR
        END IF
    NEXT m
NEXT n
PRINT "done..."
END

XBasic

Translation of: Ring

Works with: Windows XBasic

PROGRAM  "program name"
VERSION  "0.0000"

DECLARE FUNCTION  Entry ()

FUNCTION  Entry ()
  PRINT "the first 15 terms of the sequence are:"
  FOR n = 1 TO 15
    FOR m = 1 TO 4100
      pnum = 0
      FOR p = 1 TO 4100
        IF m MOD p = 0 THEN INC pnum
      NEXT p
      IF pnum = n THEN
         PRINT m;
         EXIT FOR
      END IF
    NEXT m
  NEXT n
END FUNCTION
END PROGRAM

PureBasic

Translation of: FreeBASIC

Procedure.i divisors(n)
  ;find the number of divisors of an integer
  Define.i r, i
  r = 2
  For i = 2 To n/2
    If n % i = 0: r + 1
    EndIf
  Next i
  ProcedureReturn r
EndProcedure

OpenConsole()
Define.i UPTO, i, n, nfound

UPTO = 15
i = 2
nfound = 1
Dim sfound.i(UPTO)
sfound(1) = 1

While nfound < UPTO
  n = divisors(i)
  If n <= UPTO And sfound(n) = 0:
    nfound + 1
    sfound(n) = i
  EndIf
  i + 1
Wend

Print("1 ")    ;special case
For i = 2 To UPTO
  Print(Str(sfound(i)) + " ")
Next i

PrintN(#CRLF$ + "Press ENTER to exit"): Input()
CloseConsole()

Yabasic

Translation of: FreeBASIC

UPTO = 15
i = 2
nfound = 1
dim sfound(UPTO)
sfound(1) = 1

while nfound < UPTO
    n = divisors(i)
    if n <= UPTO and sfound(n) = 0 then
        nfound = nfound + 1
        sfound(n) = i
    fi
    i = i + 1
end while

print 1, " ";    //special case
for i = 2 to UPTO
    print sfound(i), " ";
next i
print
end

sub divisors(n)
    local r, i
    
    //find the number of divisors of an integer
    r = 2
    for i = 2 to n / 2
        if mod(n, i) = 0  r = r + 1
    next i
    return r
end sub

BCPL

get "libhdr"

manifest $( LENGTH = 15 $) 

let divisors(n) = valof
$(  let count = 0 and i = 1
    while i*i <= n
    $(  if n rem i = 0 then
            count := count + (i = n/i -> 1, 2)
        i := i + 1
    $)
    resultis count
$)

let sequence(n, seq) be
$(  let found = 0 and i = 1
    for i=1 to n do seq!i := 0
    while found < n
    $(  let divs = divisors(i)
        if divs <= n & seq!divs = 0
        $(  found := found + 1
            seq!divs := i
        $)
        i := i + 1
    $)
$)

let start() be
$(  let seq = vec LENGTH
    sequence(LENGTH, seq)
    
    for i=1 to LENGTH do writef("%N ", seq!i)
    wrch('*N')
$)

Output:

1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144

C

Translation of: Go

#include <stdio.h>

#define MAX 15

int count_divisors(int n) {
    int i, count = 0;
    for (i = 1; i * i <= n; ++i) {
        if (!(n % i)) {
            if (i == n / i)
                count++;
            else
                count += 2;
        }
    }
    return count;
}

int main() {
    int i, k, n, seq[MAX];
    for (i = 0; i < MAX; ++i) seq[i] = 0;
    printf("The first %d terms of the sequence are:\n", MAX);
    for (i = 1, n = 0; n <  MAX; ++i) {
        k = count_divisors(i);
        if (k <= MAX && seq[k - 1] == 0) {
            seq[k - 1] = i;
            ++n;
        }
    }
    for (i = 0; i < MAX; ++i) printf("%d ", seq[i]);
    printf("\n");
    return 0;
}

Output:

The first 15 terms of the sequence are:
1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144 

C++

Translation of: C

#include <iostream>

#define MAX 15

using namespace std;

int count_divisors(int n) {
    int count = 0;
    for (int i = 1; i * i <= n; ++i) {
        if (!(n % i)) {
            if (i == n / i)
                count++;
            else
                count += 2;
        }
    }
    return count;
}

int main() {
    int i, k, n, seq[MAX];
    for (i = 0; i < MAX; ++i) seq[i] = 0;
    cout << "The first " << MAX << " terms of the sequence are:" << endl;
    for (i = 1, n = 0; n <  MAX; ++i) {
        k = count_divisors(i);
        if (k <= MAX && seq[k - 1] == 0) {
            seq[k - 1] = i;
            ++n;
        }
    }
    for (i = 0; i < MAX; ++i) cout << seq[i] << " ";
    cout << endl;
    return 0;
}

Output:

The first 15 terms of the sequence are:
1 2 4 6 16 12 64 24 36 48 1024 60 0 192 144 

Cowgol

include "cowgol.coh";

const AMOUNT := 15;
typedef I is uint16; 

sub divisors(n: I): (count: I) is
    var i: I := 1;
    count := 0;
    
    while i*i <= n loop
        if n%i == 0 then
            if n/i == i then
                count := count + 1;
            else
                count := count + 2;
            end if;
        end if;
        i := i + 1;
    end loop;
end sub;

var seq: I[AMOUNT+1];
MemZero(&seq as [uint8], @bytesof seq);

var found: I := 0;
var i: I := 1;

while found < AMOUNT loop
    var divs := divisors(i) as @indexof seq;
    if divs <= AMOUNT and seq[divs] == 0 then
        found := found + 1;
        seq[divs] := i;
    end if;
    i := i + 1;
end loop;

var j: @indexof seq := 1;
while j <= AMOUNT loop
    print_i16(seq[j]);
    print_char(' ');
    j := j + 1;
end loop;
print_nl();

Output:

1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144

Delphi

Works with: Delphi version 6.0

Library: SysUtils,StdCtrls

{This code would normally be in a separate library. It is provided for clarity}

function GetAllProperDivisors(N: Integer;var IA: TIntegerDynArray): integer;
{Make a list of all the "proper dividers" for N}
{Proper dividers are the of numbers the divide evenly into N}
var I: integer;
begin
SetLength(IA,0);
for I:=1 to N-1 do
 if (N mod I)=0 then
	begin
	SetLength(IA,Length(IA)+1);
	IA[High(IA)]:=I;
	end;
Result:=Length(IA);
end;


function GetAllDivisors(N: Integer;var IA: TIntegerDynArray): integer;
{Make a list of all the "proper dividers" for N, Plus N itself}
begin
Result:=GetAllProperDivisors(N,IA)+1;
SetLength(IA,Length(IA)+1);
IA[High(IA)]:=N;
end;


{-------------------------------------------------------------------------------}

procedure SequenceWithNdivisors(Memo: TMemo);
var N,N2: integer;
var IA: TIntegerDynArray;
begin
for N:=1 to 15 do
	begin
	N2:=0;
	repeat Inc(N2) until N=GetAllDivisors(N2,IA);
	Memo.Lines.Add(IntToStr(N)+' - '+IntToStr(N2));
	end;
end;

Output:

1 - 1
2 - 2
3 - 4
4 - 6
5 - 16
6 - 12
7 - 64
8 - 24
9 - 36
10 - 48
11 - 1024
12 - 60
13 - 4096
14 - 192
15 - 144

Elapsed Time: 54.950 ms.

EasyLang

func cntdiv n .
   i = 1
   while i <= sqrt n
      if n mod i = 0
         cnt += 1
         if i <> sqrt n
            cnt += 1
         .
      .
      i += 1
   .
   return cnt
.
len seq[] 15
i = 1
while n < 15
   k = cntdiv i
   if k <= 15 and seq[k] = 0
      seq[k] = i
      n += 1
   .
   i += 1
.
for v in seq[]
   print v
.

F#

This task uses Extensible Prime Generator (F#)

// Find Antı-Primes plus. Nigel Galloway: April 9th., 2019
// Increasing the value 14 will increase the number of anti-primes plus found 
let fI=primes|>Seq.take 14|>Seq.map bigint|>List.ofSeq
let N=Seq.reduce(*) fI
let fG g=Seq.unfold(fun ((n,i,e) as z)->Some(z,(n+1,i+1,(e*g)))) (1,2,g)
let fE n i=n|>Seq.collect(fun(n,e,g)->Seq.map(fun(a,c,b)->(a,c*e,g*b)) (i|>Seq.takeWhile(fun(g,_,_)->g<=n))|> Seq.takeWhile(fun(_,_,n)->n<N))
let fL=let mutable g=0 in (fun n->g<-g+1; n=g)
let n=Seq.concat(Seq.scan(fun n g->fE n (fG g)) (seq[(2147483647,1,1I)]) fI)|>List.ofSeq|>List.groupBy(fun(_,n,_)->n)|>List.sortBy(fun(n,_)->n)|>List.takeWhile(fun(n,_)->fL n)
for n,g in n do printfn "%d->%A" n (g|>List.map(fun(_,_,n)->n)|>List.min)

Output:

1->1
2->2
3->4
4->6
5->16
6->12
7->64
8->24
9->36
10->48
11->1024
12->60
13->4096
14->192
15->144
16->120
17->65536
18->180
19->262144
20->240
21->576
22->3072
23->4194304
24->360
25->1296
26->12288
27->900
28->960
29->268435456
30->720
31->1073741824
32->840
33->9216
34->196608
35->5184
36->1260
37->68719476736
38->786432
39->36864
40->1680
41->1099511627776
42->2880
43->4398046511104
44->15360
45->3600
46->12582912
47->70368744177664
48->2520
49->46656
50->6480
51->589824
52->61440
53->4503599627370496
54->6300
55->82944
56->6720
57->2359296
58->805306368
Real: 00:00:01.079, CPU: 00:00:01.080, GC gen0: 47, gen1: 0

Factor

USING: fry kernel lists lists.lazy math math.primes.factors
prettyprint sequences ;

: A005179 ( -- list )
    1 lfrom [
        1 swap '[ dup divisors length _ = ] [ 1 + ] until
    ] lmap-lazy ;

15 A005179 ltake list>array .

Output:

{ 1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144 }

FreeBASIC

#define UPTO 15

function divisors(byval n as ulongint) as uinteger
    'find the number of divisors of an integer
    dim as integer r = 2, i
    for i = 2 to n\2
        if n mod i = 0 then r += 1
    next i
    return r
end function

dim as ulongint i = 2
dim as integer n, nfound = 1, sfound(UPTO) = {1}

while nfound < UPTO
    n = divisors(i)
    if n<=UPTO andalso sfound(n)=0 then
        nfound += 1
        sfound(n) = i
    end if
    i+=1
wend

print 1;" ";    'special case
for i = 2 to UPTO
    print sfound(i);" ";
next i
print
end

Output:

 1  2  4  6  16  12  64  24  36  48  1024  60  4096  192  144

Go

package main

import "fmt"

func countDivisors(n int) int {
    count := 0
    for i := 1; i*i <= n; i++ {
        if n%i == 0 {
            if i == n/i {
                count++
            } else {
                count += 2
            }
        }
    }
    return count
}

func main() {
    const max = 15
    seq := make([]int, max)
    fmt.Println("The first", max, "terms of the sequence are:")
    for i, n := 1, 0; n < max; i++ {
        if k := countDivisors(i); k <= max && seq[k-1] == 0 {
            seq[k-1] = i
            n++
        }
    }
    fmt.Println(seq)
}

Output:

The first 15 terms of the sequence are:
[1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144]

Haskell

Without any subtlety or optimisation, but still fast at this modest scale:

import Data.List (find, group, sort)
import Data.Maybe (mapMaybe)
import Data.Numbers.Primes (primeFactors)

------------------------- A005179 ------------------------

a005179 :: [Int]
a005179 =
  mapMaybe
    ( \n ->
        find
          ((n ==) . succ . length . properDivisors)
          [1 ..]
    )
    [1 ..]

--------------------------- TEST -------------------------
main :: IO ()
main = print $ take 15 a005179

------------------------- GENERIC ------------------------

properDivisors :: Int -> [Int]
properDivisors =
  init
    . sort
    . foldr
      (flip ((<*>) . fmap (*)) . scanl (*) 1)
      [1]
    . group
    . primeFactors

Output:

[1,2,4,6,16,12,64,24,36,48,1024,60,4096,192,144]

JavaScript

Defining a generator in terms of re-usable generic functions, and taking the first 15 terms:

(() => {
    'use strict';

    // a005179 :: () -> [Int]
    const a005179 = () =>
        fmapGen(
            n => find(
                compose(
                    eq(n),
                    succ,
                    length,
                    properDivisors
                )
            )(enumFrom(1)).Just
        )(enumFrom(1));


    // ------------------------TEST------------------------
    // main :: IO ()
    const main = () =>
        console.log(
            take(15)(
                a005179()
            )
        );

    // [1,2,4,6,16,12,64,24,36,48,1024,60,4096,192,144]


    // -----------------GENERIC FUNCTIONS------------------

    // Just :: a -> Maybe a
    const Just = x => ({
        type: 'Maybe',
        Nothing: false,
        Just: x
    });

    // Nothing :: Maybe a
    const Nothing = () => ({
        type: 'Maybe',
        Nothing: true,
    });

    // Tuple (,) :: a -> b -> (a, b)
    const Tuple = a =>
        b => ({
            type: 'Tuple',
            '0': a,
            '1': b,
            length: 2
        });

    // compose (<<<) :: (b -> c) -> (a -> b) -> a -> c
    const compose = (...fs) =>
        fs.reduce(
            (f, g) => x => f(g(x)),
            x => x
        );

    // enumFrom :: Enum a => a -> [a]
    function* enumFrom(x) {
        // A non-finite succession of enumerable
        // values, starting with the value x.
        let v = x;
        while (true) {
            yield v;
            v = succ(v);
        }
    };

    // eq (==) :: Eq a => a -> a -> Bool
    const eq = a =>
        // True when a and b are equivalent in the terms
        // defined below for their shared data type.
        b => a === b;

    // find :: (a -> Bool) -> Gen [a] -> Maybe a
    const find = p => xs => {
        const mb = until(tpl => {
            const nxt = tpl[0];
            return nxt.done || p(nxt.value);
        })(
            tpl => Tuple(tpl[1].next())(
                tpl[1]
            )
        )(Tuple(xs.next())(xs))[0];
        return mb.done ? (
            Nothing()
        ) : Just(mb.value);
    }

    // fmapGen <$> :: (a -> b) -> Gen [a] -> Gen [b]
    const fmapGen = f =>
        function*(gen) {
            let v = take(1)(gen);
            while (0 < v.length) {
                yield(f(v[0]))
                v = take(1)(gen)
            }
        };

    // group :: [a] -> [[a]]
    const group = xs => {
        // A list of lists, each containing only equal elements,
        // such that the concatenation of these lists is xs.
        const go = xs =>
            0 < xs.length ? (() => {
                const
                    h = xs[0],
                    i = xs.findIndex(x => h !== x);
                return i !== -1 ? (
                    [xs.slice(0, i)].concat(go(xs.slice(i)))
                ) : [xs];
            })() : [];
        return go(xs);
    };

    // length :: [a] -> Int
    const length = xs =>
        // Returns Infinity over objects without finite
        // length. This enables zip and zipWith to choose
        // the shorter argument when one is non-finite,
        // like cycle, repeat etc
        (Array.isArray(xs) || 'string' === typeof xs) ? (
            xs.length
        ) : Infinity;

    // liftA2List :: (a -> b -> c) -> [a] -> [b] -> [c]
    const liftA2List = f => xs => ys =>
        // The binary operator f lifted to a function over two
        // lists. f applied to each pair of arguments in the
        // cartesian product of xs and ys.
        xs.flatMap(
            x => ys.map(f(x))
        );

    // mul (*) :: Num a => a -> a -> a
    const mul = a => b => a * b;

    // properDivisors :: Int -> [Int]
    const properDivisors = n =>
        // The ordered divisors of n,
        // excluding n itself.
        1 < n ? (
            sort(group(primeFactors(n)).reduce(
                (a, g) => liftA2List(mul)(a)(
                    scanl(mul)([1])(g)
                ),
                [1]
            )).slice(0, -1)
        ) : [];

    // primeFactors :: Int -> [Int]
    const primeFactors = n => {
        // A list of the prime factors of n.
        const
            go = x => {
                const
                    root = Math.floor(Math.sqrt(x)),
                    m = until(
                        ([q, _]) => (root < q) || (0 === (x % q))
                    )(
                        ([_, r]) => [step(r), 1 + r]
                    )(
                        [0 === x % 2 ? 2 : 3, 1]
                    )[0];
                return m > root ? (
                    [x]
                ) : ([m].concat(go(Math.floor(x / m))));
            },
            step = x => 1 + (x << 2) - ((x >> 1) << 1);
        return go(n);
    };

    // scanl :: (b -> a -> b) -> b -> [a] -> [b]
    const scanl = f => startValue => xs =>
        xs.reduce((a, x) => {
            const v = f(a[0])(x);
            return Tuple(v)(a[1].concat(v));
        }, Tuple(startValue)([startValue]))[1];

    // sort :: Ord a => [a] -> [a]
    const sort = xs => xs.slice()
        .sort((a, b) => a < b ? -1 : (a > b ? 1 : 0));

    // succ :: Enum a => a -> a
    const succ = x =>
        1 + x;

    // take :: Int -> [a] -> [a]
    // take :: Int -> String -> String
    const take = n =>
        // The first n elements of a list,
        // string of characters, or stream.
        xs => 'GeneratorFunction' !== xs
        .constructor.constructor.name ? (
            xs.slice(0, n)
        ) : [].concat.apply([], Array.from({
            length: n
        }, () => {
            const x = xs.next();
            return x.done ? [] : [x.value];
        }));

    // until :: (a -> Bool) -> (a -> a) -> a -> a
    const until = p => f => x => {
        let v = x;
        while (!p(v)) v = f(v);
        return v;
    };

    // MAIN ---
    return main();
})();

Output:

[1,2,4,6,16,12,64,24,36,48,1024,60,4096,192,144]

J

Rather than factoring by division, this algorithm uses a sieve to tally the factors. Of the whole numbers below ten thousand these are the smallest with n divisors. The list is fully populated through the first 15.

sieve=: 3 :0
 range=. <. + i.@:|@:-
 tally=. y#0
 for_i.#\tally do.
  j=. }:^:(y<:{:)i * 1 range >: <. y % i
  tally=. j >:@:{`[`]} tally
 end.
 /:~({./.~ {."1) tally,.i.#tally
)

|: sieve 10000
0 1 2 3 4  5  6  7  8  9 10   11 12   13  14  15  16  18  20  21   22  24   25  27  28  30  32   33   35   36   40   42   45   48   50   54   56   60   64
0 1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144 120 180 240 576 3072 360 1296 900 960 720 840 9216 5184 1260 1680 2880 3600 2520 6480 6300 6720 5040 7560

Java

Translation of: C

import java.util.Arrays;

public class OEIS_A005179 {

    static int count_divisors(int n) {
        int count = 0;
        for (int i = 1; i * i <= n; ++i) {
            if (n % i == 0) {
                if (i == n / i)
                    count++;
                else
                    count += 2;
            }
        }
        return count;
    }

    public static void main(String[] args) {
        final int max = 15;
        int[] seq = new int[max];
        System.out.printf("The first %d terms of the sequence are:\n", max);
        for (int i = 1, n = 0; n < max; ++i) {
            int k = count_divisors(i);
            if (k <= max && seq[k - 1] == 0) {        
                seq[k- 1] = i;
                n++;
            }
        }
        System.out.println(Arrays.toString(seq));
    }
}

Output:

The first 15 terms of the sequence are:
[1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144]

jq

This entry uses a streaming approach to avoid constructing unnecessary arrays.

# divisors as an unsorted stream (without calling sqrt)
def divisors:
  if . == 1 then 1
  else . as $n
  | label $out
  | range(1; $n) as $i
  | ($i * $i) as $i2
  | if $i2 > $n then break $out
    else if $i2 == $n
         then $i
         elif ($n % $i) == 0
         then $i, ($n/$i)
         else empty
	 end
    end
  end;

def count(s): reduce s as $x (0; .+1);

# smallest number with exactly n divisors
def A005179:
  . as $n
  | first( range(1; infinite) | select( count(divisors) == $n ));

# The task:
"The first 15 terms of the sequence are:",
[range(1; 16) | A005179]

Invocation: jq -ncr -f A005179.jq

Output:

The first 15 terms of the sequence are:
[1,2,4,6,16,12,64,24,36,48,1024,60,4096,192,144]

Julia

Works with: Julia version 1.2

using Primes

numfactors(n) = reduce(*, e+1 for (_,e) in factor(n); init=1)

A005179(n) = findfirst(k -> numfactors(k) == n, 1:typemax(Int))

println("The first 15 terms of the sequence are:")
println(map(A005179, 1:15))

Output:

First 15 terms of OEIS sequence A005179:
1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144

Kotlin

Translation of: Go

// Version 1.3.21

const val MAX = 15

fun countDivisors(n: Int): Int {
    var count = 0
    var i = 1
    while (i * i <= n) {
        if (n % i == 0) {
            count += if (i == n / i) 1 else 2
        }
        i++
    }
    return count
}

fun main() {
    var seq = IntArray(MAX)
    println("The first $MAX terms of the sequence are:")
    var i = 1
    var n = 0
    while (n < MAX) {
        var k = countDivisors(i)
        if (k <= MAX && seq[k - 1] == 0) {
            seq[k - 1] = i
            n++
        }
        i++
    }
    println(seq.asList())
}

Output:

The first 15 terms of the sequence are:
[1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144]

Maple

with(NumberTheory):

countDivisors := proc(x::integer)
 return numelems(Divisors(x));
end proc:

sequenceValue := proc(x::integer)
 local count:
 for count from 1 to infinity while not countDivisors(count) = x do end:
 return count;
end proc:

seq(sequenceValue(number), number = 1..15);

Output:

        1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144

Mathematica / Wolfram Language

Take[SplitBy[SortBy[{DivisorSigma[0, #], #} & /@ Range[100000], First], First][[All, 1]], 15] // Grid

Output:

1	1
2	2
3	4
4	6
5	16
6	12
7	64
8	24
9	36
10	48
11	1024
12	60
13	4096
14	192
15	144

MiniScript

divisors = function(n)
	divs = {1: 1}
	divs[n] = 1
	i = 2
	while i * i <= n
		if n % i == 0 then
			divs[i] = 1
			divs[n / i] = 1
		end if
		i += 1
	end while
	return divs.indexes
end function

counts = []
for i in range(1, 15)
	j = 1
	while divisors(j).len != i
		j += 1
	end while
	counts.push(j)
end for

print "The first 15 terms in the sequence are:"
print counts.join(", ")

Output:

The first 15 terms in the sequence are:
1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144

Miranda

main :: [sys_message]
main = [Stdout (show (take 15 a005179) ++ "\n")]

a005179 :: [num]
a005179 = map smallest_n_divisors [1..]

smallest_n_divisors :: num->num
smallest_n_divisors n = hd [i | i<-[1..]; n = #divisors i]

divisors :: num->[num]
divisors n = [d | d<-[1..n]; n mod d=0]

Output:

[1,2,4,6,16,12,64,24,36,48,1024,60,4096,192,144]

Modula-2

MODULE A005179;
FROM InOut IMPORT WriteCard, WriteLn;

CONST Amount = 15;
VAR found, i, ndivs: CARDINAL;
    A: ARRAY [1..Amount] OF CARDINAL;
    
PROCEDURE divisors(n: CARDINAL): CARDINAL;
    VAR count, i: CARDINAL;
BEGIN
    count := 0;
    i := 1;
    WHILE i*i <= n DO
        IF n MOD i = 0 THEN
            INC(count);
            IF n DIV i # i THEN
                INC(count);
            END;
        END;
        INC(i);
    END;
    RETURN count;
END divisors;

BEGIN
    FOR i := 1 TO Amount DO A[i] := 0; END;
    
    found := 0;
    i := 1;
    WHILE found < Amount DO
        ndivs := divisors(i);
        IF (ndivs <= Amount) AND (A[ndivs] = 0) THEN
            INC(found);
            A[ndivs] := i;
        END;
        INC(i);
    END;
    
    FOR i := 1 TO Amount DO
        WriteCard(A[i], 4);
        WriteLn;
    END;
END A005179.

Output:

   1
   2
   4
   6
  16
  12
  64
  24
  36
  48
1024
  60
4096
 192
 144

Nanoquery

Translation of: Java

def count_divisors(n)
        count = 0
        for (i = 1) ((i * i) <= n) (i += 1)
                if (n % i) = 0
                        if i = (n / i)
                                count += 1
                        else
                                count += 2
                        end
                end
        end
        return count
end

max = 15
seq = {0} * max
print format("The first %d terms of the sequence are:\n", max)
i = 1
for (n = 0) (n < max) (i += 1)
        k = count_divisors(i)
        if (k <= max)
                if seq[k - 1] = 0
                        seq[k - 1] = i
                        n += 1
                end
        end
end
println seq

Output:

The first 15 terms of the sequence are:
[1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144]

Nim

Translation of: Kotlin

import strformat

const MAX = 15

func countDivisors(n: int): int =
  var count = 0
  var i = 1
  while i * i <= n:
    if n mod i == 0:
      if i == n div i:
        inc count, 1
      else:
        inc count, 2
    inc i
  count

var sequence: array[MAX, int]
echo fmt"The first {MAX} terms of the sequence are:"
var i = 1
var n = 0
while n < MAX:
  var k = countDivisors(i)
  if k <= MAX and sequence[k - 1] == 0:
    sequence[k - 1] = i
    inc n
  inc i
echo sequence

Output:

The first 15 terms of the sequence are:
[1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144]

Perl

Library: ntheory

use strict;
use warnings;
use ntheory 'divisors';

print "First 15 terms of OEIS: A005179\n";
for my $n (1..15) {
    my $l = 0;
    while (++$l) {
        print "$l " and last if $n == divisors($l);
    }
}

Output:

First 15 terms of OEIS: A005179
1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144

Phix

naive

with javascript_semantics 
constant limit = 15
sequence res = repeat(0,limit)
integer found = 0, n = 1
while found<limit do
    integer k = length(factors(n,1))
    if k<=limit and res[k]=0 then
        res[k] = n
        found += 1
    end if
    n += 1
end while
printf(1,"The first %d terms are: %V\n",{limit,res})

Output:

The first 15 terms are: {1,2,4,6,16,12,64,24,36,48,1024,60,4096,192,144}

You would need something quite a bit smarter to venture over a limit of 28.

advanced

Using the various formula from the OEIS:A005179 link above.
get_primes() and product() have recently been added as new builtins, if necessary see Extensible_prime_generator and Deconvolution/2D+#Phix.

with javascript_semantics 
constant limit = iff(machine_bits()=32?58:66)
sequence found = repeat(0,limit)
integer n = 1

procedure populate_found(integer i)
    while found[i]=0 do
        integer k = length(factors(n,1))
        if k<=limit and found[k]=0 then
            found[k] = n
        end if
        n += 1
    end while
end procedure

for i=1 to limit do
    sequence f = factors(i,1)
    integer lf = length(f)
    atom ri
    if lf<=2 then                       ri = power(2,i-1)                               -- prime (or 1)
    elsif lf=3 then                     ri = power(6,f[2]-1)                            -- p^2 (eg f={1,5,25})
    elsif f[2]>2 -- (see note)
      and f[$] = power(f[2],lf-1) then  ri = power(product(get_primes(-(lf-1))),f[2]-1) -- p^k (eg f={1,3,9,27})
    elsif lf=4 then                     ri = power(2,f[3]-1)*power(3,f[2]-1)            -- p*q (eg f={1,2,3,6})
    else populate_found(i)              ri = found[i]                                   -- do the rest manually
    end if
    printf(1,"%d->%d\n",{i,ri})
end for

Note: the f[2]>2 test should really be something more like >log(get_primes(-(lf-1))[$])/log(2), apparently, but everything seems ok within the IEEE 754 53/64 bit limits this imposes. It takes longer, afaict, to print the answers than it did to calculate them, tee hee!

Output:

64-bit (as shown) manages 8 more answers than 32-bit, which as per limit halts on 58: on 32 bit the accuracy limit is 2^53, hence the result for 59, which is 2^58, would get printed wrong since the first /10 needed to print it rounds to the nearest 16 or so. It is quite probably perfectly accurate internally up to much higher limits, but proving/showing that is a bit of a problem, which would in turn probably be easiest to solve by simply rewriting this to use gmp/mpir.

1->1
2->2
3->4
4->6
5->16
6->12
7->64
8->24
9->36
10->48
11->1024
12->60
13->4096
14->192
15->144
16->120
17->65536
18->180
19->262144
20->240
21->576
22->3072
23->4194304
24->360
25->1296
26->12288
27->900
28->960
29->268435456
30->720
31->1073741824
32->840
33->9216
34->196608
35->5184
36->1260
37->68719476736
38->786432
39->36864
40->1680
41->1099511627776
42->2880
43->4398046511104
44->15360
45->3600
46->12582912
47->70368744177664
48->2520
49->46656
50->6480
51->589824
52->61440
53->4503599627370496
54->6300
55->82944
56->6720
57->2359296
58->805306368
59->288230376151711744
60->5040
61->1152921504606846976
62->3221225472
63->14400
64->7560
65->331776
66->46080

insane

A rather silly (but successful) attempt to reverse engineer all the rules up to 2000.
I got it down to just 11 of them, with only 1 being a complete fudge. Obviously, the fewer cases each covers, the less sound it is, and those mini-tables for np/p2/p3/p5 and adj are not exactly, um, scientific. Completes in about 0.1s

Library: Phix/mpfr

with javascript_semantics 
include mpfr.e
mpz r = mpz_init(),
    pn = mpz_init()
sequence rule_names = {},
         rule_counts = {}
for i=1 to 2000 do
    sequence pf = prime_factors(i,true), ri, adj
    integer lf = length(pf), np, p2, p3, p5, p, e
    string what
    if lf>10 then ?9/0 end if
    if lf<=1 then                                   what = "prime (proper rule)"
        np = 1
        adj = {i}
    elsif pf[$]=2 then                              what = "2^k (made up rule)"
        np = lf-1
        p2 = {2,4,4,4,4,4,4,8,8}[np]
        p3 = {2,2,2,2,4,4,4,4,4}[np]
        np = {2,2,3,4,4,5,6,6,7}[np]
        adj = {p2,p3}
    elsif pf[$]=3
      and pf[$-1]=2 then                            what = "2^k*3 (made up rule)"
        np = lf-1
        p2 = {3,3,4,4,4,6,6,6,6}[np]
        p3 = {2,2,3,3,3,4,4,4,4}[np]
        np = {2,3,3,4,5,5,6,7,8}[np]
        adj = {p2,p3}
    elsif lf>4
      and pf[$-1]=2 then                            what="2^k*p (made up rule)"
        np = lf-1
        adj = {0,4}
    elsif lf>4
      and pf[$]=3
      and pf[$-1]=3
      and pf[$-2]=2 then                            what="2^k*3^2*p (made up rule)"
        np = lf-4
        p3 = {3,3,3,4,4}[np]
        p5 = {2,2,2,3,3}[np]
        np = {4,5,6,6,7}[np]
        adj = {6,p3,p5}
    elsif lf>4
      and pf[$]=3
      and pf[$-2]=3
      and pf[$-4]=2 then                            what="2^k*3^3*p (made up rule)"
        np = lf-1
        adj = {6}
    elsif lf>5
      and pf[$]>3
      and pf[$-1]=3
      and pf[$-4]=3
      and pf[2]=3
      and (pf[1]=2 or pf[$]>5) then                 what="2^k*3^4*p (made up rule)"
        np = lf
        adj = {}
    elsif lf>4
      and pf[$-1]=3
      and pf[$-4]=3
      and (lf>5 or pf[$]=3) then                    what="[2^k]*3^(>=4)*p (made up rule)"
        np = lf-1
        adj = {9,pf[$]}&reverse(pf[1..$-3]) -- <bsg>
    elsif lf>=7
      and pf[$]>3
      and pf[$-1]=3
      and pf[$-2]=2 then                            what="2^k*3*p (made up rule)"
        np = lf-1
        adj = {0,4,3}
    elsif i=1440
      and pf={2,2,2,2,2,3,3,5} then                 what="1440 (complete fudge)"
        -- nothing quite like this, nothing to build any pattern from...
        np = 7
        adj = {6,5,3,2,2,2,2}
    else                                            what="general (proper rule)"
        -- (note this incorporates the p^2, (p>2)^k, p*q, and p*m*q rules)
        np = lf
        adj = {}
    end if
    ri = get_primes(-np)
    for j=1 to length(adj) do
        integer aj = adj[j]
        if aj!=0 then pf[-j] = aj end if
    end for
    for j=1 to np do
        ri[j] = {ri[j],pf[-j]-1}
    end for
 
    string short_form = ""  -- (eg "2^2*3^3" form)
    mpz_set_si(r,1)     -- (above as big int)
    for j=1 to length(ri) do
        {p, e} = ri[j]
        if length(short_form) then short_form &= "*" end if
        short_form &= sprintf("%d",p)
        if e!=1 then
            short_form &= sprintf("^%d",{e})
        end if
        mpz_ui_pow_ui(pn,p,e)
        mpz_mul(r,r,pn)
    end for
    if i<=15 or remainder(i-1,250)>=248 or i=1440 then
        string rs = mpz_get_str(r)
        if length(rs)>20 then
            rs[6..-6] = sprintf("<-- %d digits -->",length(rs)-10)
        end if
        if short_form="2^0" then short_form = "1" end if
        printf(1,"%4d : %25s %30s %s\n",{i,short_form,rs,what})
    end if
    integer k = find(what,rule_names)
    if k=0 then
        rule_names = append(rule_names,what)
        rule_counts = append(rule_counts,1)
    else
        rule_counts[k] += 1
    end if
end for
integer lr = length(rule_names)
printf(1,"\nrules(%d):\n",lr)
sequence tags = custom_sort(rule_counts, tagset(lr))
for i=1 to lr do
    integer ti = tags[-i]
    printf(1,"  %30s:%d\n",{rule_names[ti],rule_counts[ti]})
end for
{r,pn} = mpz_free({r,pn})

Output:

   1 :                         1                              1 prime (proper rule)
   2 :                         2                              2 prime (proper rule)
   3 :                       2^2                              4 prime (proper rule)
   4 :                       2*3                              6 2^k (made up rule)
   5 :                       2^4                             16 prime (proper rule)
   6 :                     2^2*3                             12 2^k*3 (made up rule)
   7 :                       2^6                             64 prime (proper rule)
   8 :                     2^3*3                             24 2^k (made up rule)
   9 :                   2^2*3^2                             36 general (proper rule)
  10 :                     2^4*3                             48 general (proper rule)
  11 :                      2^10                           1024 prime (proper rule)
  12 :                   2^2*3*5                             60 2^k*3 (made up rule)
  13 :                      2^12                           4096 prime (proper rule)
  14 :                     2^6*3                            192 general (proper rule)
  15 :                   2^4*3^2                            144 general (proper rule)
 249 :                  2^82*3^2    43521<-- 16 digits -->22336 general (proper rule)
 250 :             2^4*3^4*5^4*7                        5670000 general (proper rule)
 499 :                     2^498   81834<-- 140 digits -->97344 prime (proper rule)
 500 :          2^4*3^4*5^4*7*11                       62370000 general (proper rule)
 749 :                 2^106*3^6    59143<-- 25 digits -->22656 general (proper rule)
 750 :        2^4*3^4*5^4*7^2*11                      436590000 general (proper rule)
 999 :          2^36*3^2*5^2*7^2                757632231014400 general (proper rule)
1000 :       2^4*3^4*5^4*7*11*13                      810810000 general (proper rule)
1249 :                    2^1248   48465<-- 366 digits -->22656 prime (proper rule)
1250 :        2^4*3^4*5^4*7^4*11                    21392910000 general (proper rule)
1440 :    2^5*3^4*5^2*7*11*13*17                     1102701600 1440 (complete fudge)
1499 :                    2^1498   87686<-- 441 digits -->37344 prime (proper rule)
1500 :     2^4*3^4*5^4*7^2*11*13                     5675670000 general (proper rule)
1749 :             2^52*3^10*5^2    66483<-- 12 digits -->57600 general (proper rule)
1750 :        2^6*3^4*5^4*7^4*11                    85571640000 general (proper rule)
1999 :                    2^1998   28703<-- 592 digits -->57344 prime (proper rule)
2000 :    2^4*3^4*5^4*7*11*13*17                    13783770000 general (proper rule)

rules(11):
           general (proper rule):1583
             prime (proper rule):304
            2^k*p (made up rule):59
          2^k*3*p (made up rule):9
        2^k*3^3*p (made up rule):9
              2^k (made up rule):9
            2^k*3 (made up rule):9
  [2^k]*3^(>=4)*p (made up rule):8
        2^k*3^2*p (made up rule):5
        2^k*3^4*p (made up rule):4
           1440 (complete fudge):1

PROMAL

Builds a table of divisor counts wihout division.

;;; Find the smallest number with n divisors, n in 1..15

PROGRAM SmallestN
INCLUDE LIBRARY

CON WORD maxDivisors =   15 ; number of sequence elements to find
CON WORD maxNUmber   = 6000 ; maximum number we will consider

WORD dc [ maxNumber + 1 ]
WORD firstWithDivisors [ maxDivisors + 1 ]
WORD found
WORD divisors
WORD d
WORD n
WORD j

BEGIN
; compute a table of divisor counts
FOR n = 1 TO maxNumber
  dc[ n ] = 0
FOR n = 1 TO maxNumber
  j = n
  WHILE j <= maxNumber
    dc[ j ] = dc[ j ] + 1
    j = j + n
; find the first number wih the required divisor counts
FOR n = 0 TO maxDivisors
  firstWithDivisors[ n ] = 0
found = 0
n = 0
WHILE found < maxDivisors
  n = n + 1
  divisors = dc[ n ]
  IF divisors <= maxDivisors
    IF firstWithDivisors[ divisors ] = 0
      ; first number with this number of divisors
      found = found + 1
      firstWithDivisors[ divisors ] = n
FOR d = 1 TO maxDivisors
  OUTPUT " #W", firstWithDivisors[ d ]
END

Output:

 1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144

Python

Procedural

def divisors(n):
    divs = [1]
    for ii in range(2, int(n ** 0.5) + 3):
        if n % ii == 0:
            divs.append(ii)
            divs.append(int(n / ii))
    divs.append(n)
    return list(set(divs))


def sequence(max_n=None):
    n = 0
    while True:
        n += 1
        ii = 0
        if max_n is not None:
            if n > max_n:
                break
        while True:
            ii += 1
            if len(divisors(ii)) == n:
                yield ii
                break


if __name__ == '__main__':
    for item in sequence(15):
        print(item)

Output:

1
2
4
6
16
12
64
24
36
48
1024
60
4096
192
144

Functional

Aiming for functional transparency, speed of writing, high levels of code reuse, and ease of maintenance.

Not optimized, but still fast at this modest scale.

'''Smallest number with exactly n divisors'''

from itertools import accumulate, chain, count, groupby, islice, product
from functools import reduce
from math import sqrt, floor
from operator import mul


# a005179 :: () -> [Int]
def a005179():
    '''Integer sequence: smallest number with exactly n divisors.'''
    return (
        next(
            x for x in count(1)
            if n == 1 + len(properDivisors(x))
        ) for n in count(1)
    )


# --------------------------TEST---------------------------
# main :: IO ()
def main():
    '''First 15 terms of a005179'''
    print(main.__doc__ + ':\n')
    print(
        take(15)(
            a005179()
        )
    )


# -------------------------GENERIC-------------------------

# properDivisors :: Int -> [Int]
def properDivisors(n):
    '''The ordered divisors of n, excluding n itself.
    '''
    def go(a, x):
        return [a * b for a, b in product(
            a,
            accumulate(chain([1], x), mul)
        )]
    return sorted(
        reduce(go, [
            list(g) for _, g in groupby(primeFactors(n))
        ], [1])
    )[:-1] if 1 < n else []


# primeFactors :: Int -> [Int]
def primeFactors(n):
    '''A list of the prime factors of n.
    '''
    def f(qr):
        r = qr[1]
        return step(r), 1 + r

    def step(x):
        return 1 + (x << 2) - ((x >> 1) << 1)

    def go(x):
        root = floor(sqrt(x))

        def p(qr):
            q = qr[0]
            return root < q or 0 == (x % q)

        q = until(p)(f)(
            (2 if 0 == x % 2 else 3, 1)
        )[0]
        return [x] if q > root else [q] + go(x // q)

    return go(n)


# take :: Int -> [a] -> [a]
# take :: Int -> String -> String
def take(n):
    '''The prefix of xs of length n,
       or xs itself if n > length xs.
    '''
    return lambda xs: (
        xs[0:n]
        if isinstance(xs, (list, tuple))
        else list(islice(xs, n))
    )


# until :: (a -> Bool) -> (a -> a) -> a -> a
def until(p):
    '''The result of repeatedly applying f until p holds.
       The initial seed value is x.
    '''
    def go(f, x):
        v = x
        while not p(v):
            v = f(v)
        return v
    return lambda f: lambda x: go(f, x)


# MAIN ---
if __name__ == '__main__':
    main()

Output:

[1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144]

Quackery

factors is defined at Factors of an integer.

 [ 0
   [ 1+ 2dup 
     factors size = 
     until ] 
   nip ]            is nfactors ( n --> n )

 15 times [ i^ 1+ nfactors echo sp ]

Output:

1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144

R

Could probably be speeded up by caching the results of divisorCount, but it's quick enough already. Not to mention, delightfully short.

#Need to add 1 to account for skipping n. Not the most efficient way to count divisors, but quite clear.
divisorCount <- function(n) length(Filter(function(x) n %% x == 0, seq_len(n %/% 2))) + 1
smallestWithNDivisors <- function(n)
{
  i <- 1
  while(divisorCount(i) != n) i <- i + 1
  i
}
print(sapply(1:15, smallestWithNDivisors))

Output:

[1]    1    2    4    6   16   12   64   24   36   48 1024   60 4096  192  144

Raku

(formerly Perl 6)

Works with: Rakudo version 2019.03

sub div-count (\x) {
    return 2 if x.is-prime;
    +flat (1 .. x.sqrt.floor).map: -> \d {
        unless x % d { my \y = x div d; y == d ?? y !! (y, d) }
    }
}

my $limit = 15;

put "First $limit terms of OEIS:A005179";
put (1..$limit).map: -> $n { first { $n == .&div-count }, 1..Inf };

Output:

First 15 terms of OEIS:A005179
1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144

REXX

some optimization

/*REXX program finds and displays  the   smallest number   with  exactly   N   divisors.*/
parse arg N .                                    /*obtain optional argument from the CL.*/
if N=='' | N==","  then N= 15                    /*Not specified?  Then use the default.*/
say '──divisors──  ──smallest number with N divisors──' /*display title for the numbers.*/
@.=                                              /*the  @  array is used for memoization*/
    do i=1  for N;     z= 1  +  (i\==1)          /*step through a number of divisors.   */
       do j=z  by z                              /*now, search for a number that ≡ #divs*/
       if @.j==.  then iterate                   /*has this number already been found?  */
       d= #divs(j);    if d\==i  then iterate    /*get # divisors;  Is not equal?  Skip.*/
       say center(i, 12)  right(j, 19)           /*display the #divs and the smallest #.*/
       @.j= .                                    /*mark as having found #divs for this J*/
       leave                                     /*found a number, so now get the next I*/
       end   /*j*/
    end      /*i*/
exit                                             /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
#divs: procedure; parse arg x 1 y                /*X and Y:  both set from 1st argument.*/
       if x<7  then do; if x<3   then return x   /*handle special cases for one and two.*/
                        if x<5   then return x-1 /*   "      "      "    "  three & four*/
                        if x==5  then return 2   /*   "      "      "    "  five.       */
                        if x==6  then return 4   /*   "      "      "    "  six.        */
                    end
       odd= x // 2                               /*check if   X   is  odd  or not.      */
       if odd  then      #= 1                    /*Odd?   Assume  Pdivisors  count of 1.*/
               else do;  #= 3;   y= x%2;  end    /*Even?     "        "        "    " 3.*/
                                                 /* [↑]   start with known num of Pdivs.*/
                  do k=3  for x%2-3  by 1+odd  while k<y  /*for odd numbers, skip evens.*/
                  if x//k==0  then do            /*if no remainder, then found a divisor*/
                                   #=#+2;  y=x%k /*bump  #  Pdivs,  calculate limit  Y. */
                                   if k>=y  then do;  #= #-1;  leave;  end      /*limit?*/
                                   end                                          /*  ___ */
                              else if k*k>x  then leave        /*only divide up to √ x  */
                  end   /*k*/                    /* [↑]  this form of DO loop is faster.*/
       return #+1                                /*bump "proper divisors" to "divisors".*/

output   when using the default input:

──divisors──  ──smallest number with N divisors──
     1                         1
     2                         2
     3                         4
     4                         6
     5                        16
     6                        12
     7                        64
     8                        24
     9                        36
     10                       48
     11                     1024
     12                       60
     13                     4096
     14                      192
     15                      144

with memorization

/*REXX program finds and displays  the   smallest number   with  exactly   N   divisors.*/
parse arg N lim .                                /*obtain optional argument from the CL.*/
if   N=='' |   N==","  then   N=      15         /*Not specified?  Then use the default.*/
if lim=='' | lim==","  then lim= 1000000         /* "      "         "   "   "     "    */
say '──divisors──  ──smallest number with N divisors──' /*display title for the numbers.*/
@.=.                                             /*the  @  array is used for memoization*/
    do i=1  for N;      z= 1  +  (i\==1)         /*step through a number of divisors.   */
       do j=z  by z                              /*now, search for a number that ≡ #divs*/
       if @.j\==.  then if @.j\==i  then iterate /*if already been found, is it THE one?*/
       d= #divs(j);       if j<lim  then @.j= d  /*get # divisors; if not too high, save*/
       if d\==i    then iterate                  /*Is d ¬== i?     Then skip this number*/
       say center(i, 12)  right(j, 19)           /*display the #divs and the smallest #.*/
       @.j= d                                    /*mark as having found #divs for this J*/
       leave                                     /*found a number, so now get the next I*/
       end   /*j*/
    end      /*i*/
exit                                             /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
#divs: procedure; parse arg x 1 y                /*X and Y:  both set from 1st argument.*/
       if x<7  then do; if x<3   then return x   /*handle special cases for one and two.*/
                        if x<5   then return x-1 /*   "      "      "    "  three & four*/
                        if x==5  then return 2   /*   "      "      "    "  five.       */
                        if x==6  then return 4   /*   "      "      "    "  six.        */
                    end
       odd= x // 2                               /*check if   X   is  odd  or not.      */
       if odd  then      #= 1                    /*Odd?   Assume  Pdivisors  count of 1.*/
               else do;  #= 3;   y= x%2;  end    /*Even?     "        "        "    " 3.*/
                                                 /* [↑]   start with known num of Pdivs.*/
                  do k=3  for x%2-3  by 1+odd  while k<y  /*for odd numbers, skip evens.*/
                  if x//k==0  then do            /*if no remainder, then found a divisor*/
                                   #=#+2;  y=x%k /*bump  #  Pdivs,  calculate limit  Y. */
                                   if k>=y  then do;  #= #-1;  leave;  end      /*limit?*/
                                   end                                          /*  ___ */
                              else if k*k>x  then leave        /*only divide up to √ x  */
                  end   /*k*/                    /* [↑]  this form of DO loop is faster.*/
       return #+1                                /*bump "proper divisors" to "divisors".*/

output   is identical to the 1^st REXX version.

Ring

load "stdlib.ring"

see "working..." + nl
see "the first 15 terms of the sequence are:" + nl

for n = 1 to 15
    for m = 1 to 4100
        pnum = 0
        for p = 1 to 4100
            if (m % p = 0)
               pnum = pnum + 1
            ok
        next
        if pnum = n
           see "" + m + " "
           exit  
        ok
     next
next

see nl + "done..." + nl

Output:

working...
the first 15 terms of the sequence are:
1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144
done...

RPL

Works with: HP version 49g

≪ {1} 
   2 ROT FOR j
      1
      DO 1 + UNTIL DUP DIVIS SIZE j == END
      +
   NEXT 
≫ 'TASK' STO

15 TASK

Output:

1: {1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144}

Runs in 13 minutes on an HP-50g.

Faster implementation

With the hint given in the OEIS page that a(p)=2^(p-1) when p is prime:

≪ {1} 
   2 ROT FOR j
      IF j ISPRIME? THEN 2 j 1 - ^ 
      ELSE
        1 DO 1 + UNTIL DUP DIVIS SIZE j == END 
      END
      +
   NEXT 
≫ 'TASK' STO

the same result is obtained in less than a minute.

Ruby

require 'prime'
 
def num_divisors(n)
  n.prime_division.inject(1){|prod, (_p,n)| prod *= (n + 1) } 
end

def first_with_num_divs(n)
  (1..).detect{|i| num_divisors(i) == n }
end

p (1..15).map{|n| first_with_num_divs(n) }

Output:

[1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144]

Rust

use itertools::Itertools;

const PRIMES: [u64; 15] = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47];
const MAX_DIVISOR: usize = 64;

struct DivisorSeq {
    max_number_of_divisors: u64,
    index: u64,
}

impl DivisorSeq {
    fn new(max_number_of_divisors: u64) -> DivisorSeq {
        DivisorSeq {
            max_number_of_divisors,
            index: 1,
        }
    }
}

impl Iterator for DivisorSeq {
    type Item = u64;

    fn next(&mut self) -> Option<u64> {
        if self.max_number_of_divisors < self.index {
            return None;
        }
        #[allow(unused_mut)]
        let mut result: u64;
        let divisors_of_divisor = get_divisors(self.index);
        match divisors_of_divisor.len() {
            1 | 2 => {
                // when # divisors is a prime
                result = 2_u64.pow(self.index as u32 - 1);
                self.index += 1;
            }
            3 => {
                // when # divisors is a prime square
                result = 6_u64.pow(divisors_of_divisor[1] as u32 - 1);
                self.index += 1;
            }
            4 => {
                // when # divisors is a prime * non-prime
                result = 2_u64.pow(divisors_of_divisor[2] as u32 - 1)
                    * 3_u64.pow(divisors_of_divisor[1] as u32 - 1);
                self.index += 1;
            }
            8 if divisors_of_divisor
                .iter()
                .filter(|x| PRIMES.contains(x))
                .count()
                == 3 =>
            {
                // sphenic numbers, aka p*m*q, where, p, m and q are prime
                let first_primes = divisors_of_divisor
                    .iter()
                    .filter(|x| PRIMES.contains(x))
                    .collect::<Vec<_>>();
                result = 2_u64.pow(*first_primes[2] as u32 - 1)
                    * 3_u64.pow(*first_primes[1] as u32 - 1)
                    * 5_u64.pow(*first_primes[0] as u32 - 1);
                self.index += 1;
            }
            _ => {
                // brute force and slow: iterates over the numbers to find
                // one with the appropriate number of divisors
                let mut x: u64 = 1;
                loop {
                    if get_divisors(x).len() as u64 == self.index {
                        break;
                    }
                    x += 1;
                }

                result = x;
                self.index += 1;
            }
        }
        Some(result)
    }
}
/// Gets all divisors of a number
fn get_divisors(n: u64) -> Vec<u64> {
    let mut results = Vec::new();

    for i in 1..(n / 2 + 1) {
        if n % i == 0 {
            results.push(i);
        }
    }
    results.push(n);
    results
}

fn main() {
    // simple version using factorizing numbers
    // with rules applied from A005179 so speed up
    // but as such it is slow in some cases, e.g for 52
    let seq_iter = DivisorSeq::new(64);
    println!("Simple method with rules");
    println!("# divisors     Smallest number");
    for (i, x) in seq_iter.enumerate() {
        println!("{:>10}{:20}", i + 1, x);
    }

    // more advanced version using calculations based on number of
    // prime factors and their exponent

    // load initial result table with an initial value of 2**n for each item
    let mut min_numbers = vec![0_u64; MAX_DIVISOR];
    (0_usize..MAX_DIVISOR).for_each(|n| min_numbers[n] = 2_u64.pow(n as u32));

    let prime_list = (1..15).map(|i| PRIMES[0..=i].to_vec()).collect::<Vec<_>>();

    for pl in prime_list.iter() {
        // calculate the max exponent a prime can get in a given prime-list
        // to be able to produce the desired number of divisors
        let max_exponent = 1 + MAX_DIVISOR as u32 / 2_u32.pow(pl.len() as u32 - 1);

        // create a combination of exponents using cartesian product
        let exponents = (1_usize..=pl.len())
            .map(|_| 1_u32..=max_exponent)
            .multi_cartesian_product()
            .filter(|elt| {
                let mut prev = None::<&u32>;
                elt.iter().all(|x| match prev {
                    Some(n) if x > n => false,
                    _ => {
                        prev = Some(x);
                        true
                    }
                })
            });

        // iterate throught he exponent combinations
        for exp in exponents {
            // calculate the number of divisors using the formula
            // given primes of p, q, r
            // and exponents of a1, a2, a3
            // the # divisors is (a1+1)* (a2+1) *(a3+1)
            let num_of_divisors = exp.iter().map(|x| x + 1).product::<u32>() as usize;

            // and calculate the number with those primes and the given exponent set
            let num = pl.iter().zip(exp.iter()).fold(1_u64, |mut acc, (p, e)| {
                // check for overflow if numbers won't fit into u64
                acc = match acc.checked_mul(p.pow(*e)) {
                    Some(z) => z,
                    _ => 0,
                };
                acc
            });

            // finally, if the number is less than what we have so far in the result table
            // replace the result table with the smaller number
            if num > 0
                && min_numbers.len() >= num_of_divisors
                && min_numbers[num_of_divisors - 1] > num
            {
                min_numbers[num_of_divisors - 1] = num;
            }
        }
    }

    println!("Advanced method");
    println!("# divisors     Smallest number");
    for (i, x) in min_numbers.iter().enumerate() {
        println!("{:>10}{:20}", i + 1, x);
    }
}

Output:

Simple method with rules
# divisors     Smallest number
         1                   1
         2                   2
         3                   4
         4                   6
         5                  16
         6                  12
         7                  64
         8                  24
         9                  36
        10                  48
        11                1024
        12                  60
        13                4096
        14                 192
        15                 144
        16                 120
        17               65536
        18                 180
        19              262144
        20                 240
        21                 576
        22                3072
        23             4194304
        24                 360
        25                1296
        26               12288
        27                2304
        28                 960
        29           268435456
        30                 720
        31          1073741824
        32                 840
        33                9216
        34              196608
        35                5184
        36                1260
        37         68719476736
        38              786432
        39               36864
        40                1680
        41       1099511627776
        42                2880
        43       4398046511104
        44               15360
        45                3600
        46            12582912
        47      70368744177664
        48                2520
        49               46656
        50                6480
        51              589824
        52               61440
        53    4503599627370496
        54                6300
        55               82944
        56                6720
        57             2359296
        58           805306368
        59  288230376151711744
        60                5040
        61 1152921504606846976
        62          3221225472
        63               14400
        64                7560

SETL

program smallest_number_with_exactly_n_divisors;
    print([a(n) : n in [1..15]]);

    proc a(n);
        return [i +:= 1 : until n = #[d : d in [1..i] | i mod d=0]](i);
    end proc;
end program;

Output:

[1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144]

Sidef

func n_divisors(n) {
    1..Inf -> first_by { .sigma0 == n }
}

say 15.of { n_divisors(_+1) }

Output:

[1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144]

More efficient solution:

func n_divisors(threshold, least_solution = Inf, k = 1, max_a = Inf, solutions = 1, n = 1) {

    if (solutions == threshold) {
        return n
    }

    if (solutions > threshold) {
        return least_solution
    }

    var p = k.prime

    for a in (1 .. max_a) {
        n *= p
        break if (n > least_solution)
        least_solution = __FUNC__(threshold, least_solution, k+1, a, solutions * (a + 1), n)
    }

    return least_solution
}

say n_divisors(60)     #=> 5040
say n_divisors(1000)   #=> 810810000

Swift

// See https://en.wikipedia.org/wiki/Divisor_function
func divisorCount(number: Int) -> Int {
    var n = number
    var total = 1
    // Deal with powers of 2 first
    while n % 2 == 0 {
        total += 1
        n /= 2
    }
    // Odd prime factors up to the square root
    var p = 3
    while p * p <= n {
        var count = 1
        while n % p == 0 {
            count += 1
            n /= p
        }
        total *= count
        p += 2
    }
    // If n > 1 then it's prime
    if n > 1 {
        total *= 2
    }
    return total
}

let limit = 15
var sequence = Array(repeating: 0, count: limit)
var count = 0
var n = 1
while count < limit {
    let divisors = divisorCount(number: n)
    if divisors <= limit && sequence[divisors - 1] == 0 {
        sequence[divisors - 1] = n
        count += 1
    }
    n += 1
}
for n in sequence {
    print(n, terminator: " ")
}
print()

Output:

1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144 

Tcl

proc divCount {n} {
    set cnt 0
    for {set d 1} {($d * $d) <= $n} {incr d} {
        if {0 == ($n % $d)} {
            incr cnt
            if {$d < ($n / $d)} {
                incr cnt
            }
        }
    }
    return $cnt
}

proc A005179 {n} {
    if {$n >= 1} {
        for {set k 1} {1} {incr k} {
            if {$n == [divCount $k]} {
                return $k
            }
        }
    }
    return 0
}

proc show {func lo hi} {
    puts "${func}($lo..$hi) ="
    for {set n $lo} {$n <= $hi} {incr n} {
        puts -nonewline " [$func $n]"
    }
    puts ""
}

show A005179 1 15

Output:

A005179(1..15) =
 1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144

Wren

Library: Wren-math

import "./math" for Int

var limit = 22
var numbers = List.filled(limit, 0)
var i = 1
while (true) {
    var nd = Int.divisors(i).count
    if (nd <= limit && numbers[nd-1] == 0) {
        numbers[nd-1] = i
        if (numbers.all { |n| n > 0 }) break
    }
    i = i + 1
}
System.print("The first %(limit) terms are:")
System.print(numbers)

Output:

The first 22 terms are:
[1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144, 120, 65536, 180, 262144, 240, 576, 3072]

XPL0

func Divisors(N); \Return number of divisors of N
int  N, Count, D;
[Count:= 0;
for D:= 1 to N do
    if rem(N/D) = 0 then Count:= Count+1;
return Count;
];

int N, AN;
[for N:= 1 to 15 do
    [AN:= 0;
    repeat AN:= AN+1 until Divisors(AN) = N;
    IntOut(0, AN);  ChOut(0, ^ );
    ];
]

Output:

1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144 

zkl

fcn countDivisors(n)
   { [1.. n.toFloat().sqrt()].reduce('wrap(s,i){ s + (if(0==n%i) 1 + (i!=n/i)) },0) }
A005179w:=(1).walker(*).tweak(fcn(n){
   var N=0,cache=Dictionary();
   if(cache.find(n)) return(cache.pop(n));	// prune
   while(1){ 
      if(n == (d:=countDivisors(N+=1))) return(N);
      if(n<d and not cache.find(d)) cache[d]=N;
   }
});

N:=15;
println("First %d terms of OEIS:A005179".fmt(N));
A005179w.walk(N).concat(" ").println();

Output:

First 15 terms of OEIS:A005179
1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144