# Neighbour primes

Find and show primes p such that p*q+2 is prime, where q is next prime after p and p < 500

Neighbour primes is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

## 11l

Translation of: Python
```F isPrime(n)
L(i) 2 .. Int(n ^ 0.5)
I n % i == 0
R 0B
R 1B

print(‘p        q       pq+2’)
print(‘-----------------------’)
L(p) 2..498
I !isPrime(p)
L.continue
V q = p + 1
L !isPrime(q)
q++
I !isPrime(2 + p * q)
L.continue
print(p" \t "q" \t "(2 + p * q))```
Output:
```p        q       pq+2
-----------------------
3 	 5 	 17
5 	 7 	 37
7 	 11 	 79
13 	 17 	 223
19 	 23 	 439
67 	 71 	 4759
149 	 151 	 22501
179 	 181 	 32401
229 	 233 	 53359
239 	 241 	 57601
241 	 251 	 60493
269 	 271 	 72901
277 	 281 	 77839
307 	 311 	 95479
313 	 317 	 99223
397 	 401 	 159199
401 	 409 	 164011
419 	 421 	 176401
439 	 443 	 194479
487 	 491 	 239119
```

## ALGOL 68

Very similar to The ALGOL 68 sample in the Special neighbor primes task

```BEGIN  # find adjacent primes p1, p2 such that p1*p2 + 2 s also prime    #
INT max prime = 500;
[]BOOL prime    = PRIMESIEVE ( ( max prime * max prime ) + 2 );      # sieve the primes to max prime ^ 2 + 2    #
[]INT low prime = EXTRACTPRIMESUPTO max prime FROMPRIMESIEVE prime;  # get a list of the primes up to max prime #
# find the adjacent primes p1, p2 such that p1*p2 + 2 is prime       #
FOR i TO UPB low prime - 1 DO
IF   INT p1 p2 plus 2 = ( low prime[ i ] * low prime[ i + 1 ] ) + 2;
prime[ p1 p2 plus 2 ]
THEN print( ( "(",         whole( low prime[ i     ], -3 )
, " *",        whole( low prime[ i + 1 ], -3 )
, " ) + 2 = ", whole( p1 p2 plus 2,       -6 )
, newline
)
)
FI
OD
END```
Output:
```(  3 *  5 ) + 2 =     17
(  5 *  7 ) + 2 =     37
(  7 * 11 ) + 2 =     79
( 13 * 17 ) + 2 =    223
( 19 * 23 ) + 2 =    439
( 67 * 71 ) + 2 =   4759
(149 *151 ) + 2 =  22501
(179 *181 ) + 2 =  32401
(229 *233 ) + 2 =  53359
(239 *241 ) + 2 =  57601
(241 *251 ) + 2 =  60493
(269 *271 ) + 2 =  72901
(277 *281 ) + 2 =  77839
(307 *311 ) + 2 =  95479
(313 *317 ) + 2 =  99223
(397 *401 ) + 2 = 159199
(401 *409 ) + 2 = 164011
(419 *421 ) + 2 = 176401
(439 *443 ) + 2 = 194479
(487 *491 ) + 2 = 239119
```

## ALGOL W

```begin % find some primes where ( p*q ) + 2 is also a prime ( where p and q are adjacent primes ) %
% sets p( 1 :: n ) to a sieve of primes up to n %
procedure sieve ( logical array p( * ) ; integer value n ) ;
begin
p( 1 ) := false; p( 2 ) := true;
for i := 3 step 2 until n do p( i ) := true;
for i := 4 step 2 until n do p( i ) := false;
for i := 3 step 2 until truncate( sqrt( n ) ) do begin
integer ii; ii := i + i;
if p( i ) then for np := i * i step ii until n do p( np ) := false
end for_i ;
end sieve ;
integer MAX_NUMBER, MAX_PRIME;
MAX_NUMBER := 500;
MAX_PRIME  := MAX_NUMBER * MAX_NUMBER;
begin
logical array prime( 1 :: MAX_PRIME );
integer       pCount, thisPrime, nextPrime;
% sieve the primes to MAX_PRIME %
sieve( prime, MAX_PRIME );
% find the neighbour primes %
pCount    := 0;
thisPrime := 2; % 2 is the lowest prime %
while thisPrime > 0 do begin
% find the next prime after this one %
nextPrime := thisPrime + 1;
while nextPrime <= MAX_NUMBER and not prime( nextPrime ) do nextPrime := nextPrime + 1;
if nextPrime > MAX_NUMBER then thisPrime := 0
else begin
if prime( ( thisPrime * nextPrime ) + 2 ) then begin
% have another neighbour prime %
writeon( i_w := 1, s_w := 0, " ", thisPrime );
pCount := pCount + 1
end if_prime__thisPrime_x_nextPrime_plus_2 ;
thisPrime := nextPrime
end if_nextPrime_gt_MAX_NUMBER__
end while_thisPrime_gt_0 ;
write( i_w := 1, s_w := 0, "Found ", pCount, " neighbour primes up to 500" )
end
end.```
Output:
``` 3 5 7 13 19 67 149 179 229 239 241 269 277 307 313 397 401 419 439 487
Found 20 neighbour primes up to 500
```

## AppleScript

```on isPrime(n)
if (n < 6) then return ((n > 1) and (n is not 4))
if ((n mod 2 = 0) or (n mod 3 = 0) or (n mod 5 = 0)) then return false
repeat with i from 7 to (n ^ 0.5) div 1 by 30
if (n mod i = 0) or (n mod (i + 4) = 0) or (n mod (i + 6) = 0) or (n mod (i + 10) = 0) or ¬
(n mod (i + 12) = 0) or (n mod (i + 16) = 0) or (n mod (i + 22) = 0) or (n mod (i + 24) = 0) then ¬
return false
end repeat

return true
end isPrime

on neighbourPrimes(max)
set output to {}

repeat with p from 3 to max by 2
if (isPrime(p)) then
set q to p + 2
repeat until (isPrime(q))
set q to q + 2
end repeat
if (isPrime(p * q + 2)) then set end of output to p
end if
end repeat

return output
end neighbourPrimes

neighbourPrimes(499)
```
Output:
```{3, 5, 7, 13, 19, 67, 149, 179, 229, 239, 241, 269, 277, 307, 313, 397, 401, 419, 439, 487}
```

## Arturo

```primesUpTo500: select 1..500 => prime?

print "--------------------"
i: 0
while [i < dec size primesUpTo500][
p: primesUpTo500\[i]
q: primesUpTo500\[i+1]
if prime? 2 + p * q [
prints pad to :string p 5
prints pad to :string q 5
print pad to :string 2 + p * q 8
]
i: i + 1
]
```
Output:
```    p    q   p*q+2
--------------------
3    5      17
5    7      37
7   11      79
13   17     223
19   23     439
67   71    4759
149  151   22501
179  181   32401
229  233   53359
239  241   57601
241  251   60493
269  271   72901
277  281   77839
307  311   95479
313  317   99223
397  401  159199
401  409  164011
419  421  176401
439  443  194479
487  491  239119```

## AWK

```# syntax: GAWK -f NEIGHBOUR_PRIMES.AWK
BEGIN {
print("   p    q  p*q+2")
print("---- ---- ------")
start = 1
stop = 499
for (p=start; p<=stop; p++) {
if (!is_prime(p)) { continue }
q = p + 1
while (!is_prime(q)) {
q++
}
if (!is_prime(p*q+2)) { continue }
printf("%4d %4d %6d\n",p,q,p*q+2)
count++
}
printf("Neighbour primes %d-%d: %d\n",start,stop,count)
exit(0)
}
function is_prime(x,  i) {
if (x <= 1) {
return(0)
}
for (i=2; i<=int(sqrt(x)); i++) {
if (x % i == 0) {
return(0)
}
}
return(1)
}
```
Output:
```   p    q  p*q+2
---- ---- ------
3    5     17
5    7     37
7   11     79
13   17    223
19   23    439
67   71   4759
149  151  22501
179  181  32401
229  233  53359
239  241  57601
241  251  60493
269  271  72901
277  281  77839
307  311  95479
313  317  99223
397  401 159199
401  409 164011
419  421 176401
439  443 194479
487  491 239119
Neighbour primes 1-499: 20
```

## BASIC

### BASIC256

```function isPrime(v)
if v < 2 then return False
if v mod 2 = 0 then return v = 2
if v mod 3 = 0 then return v = 3
d = 5
while d * d <= v
if v mod d = 0 then return False else d += 2
end while
return True
end function

print "p        q        pq+2"
print "------------------------"
for p = 2 to 499
if not isPrime(p) then continue for
q = p + 1
while Not isPrime(q)
q += 1
end while
if not isPrime(2 + p*q) then continue for
print p; chr(9); q; chr(9); 2+p*q
next p
end```

### PureBasic

```Procedure isPrime(v.i)
If     v <= 1    : ProcedureReturn #False
ElseIf v < 4     : ProcedureReturn #True
ElseIf v % 2 = 0 : ProcedureReturn #False
ElseIf v < 9     : ProcedureReturn #True
ElseIf v % 3 = 0 : ProcedureReturn #False
Else
Protected r = Round(Sqr(v), #PB_Round_Down)
Protected f = 5
While f <= r
If v % f = 0 Or v % (f + 2) = 0
ProcedureReturn #False
EndIf
f + 6
Wend
EndIf
ProcedureReturn #True
EndProcedure

OpenConsole()
PrintN("p       q       pq+2")
PrintN("----------------------")
For p.i = 2 To 499
If Not isPrime(p)
Continue
EndIf
q = p + 1
While Not isPrime(q)
q + 1
Wend
If Not isPrime(2 + p*q)
Continue
EndIf
PrintN(Str(p) + #TAB\$ + Str(q) + #TAB\$ + Str(2+p*q))
Next p
PrintN(#CRLF\$ + "--- terminado, pulsa RETURN---"): Input()
CloseConsole()```

### Yabasic

```sub isPrime(v)
if v < 2 then return False : fi
if mod(v, 2) = 0 then return v = 2 : fi
if mod(v, 3) = 0 then return v = 3 : fi
d = 5
while d * d <= v
if mod(v, d) = 0 then return False else d = d + 2 : fi
wend
return True
end sub

print "p       q       pq+2"
print "----------------------"
for p = 2 to 499
if not isPrime(p) continue
q = p + 1
while not isPrime(q)
q = q + 1
wend
if not isPrime(2 + p*q) continue
print p, chr\$(9), q, chr\$(9), 2+p*q
next p
end```

## C#

How about some other offsets besides `+ 2` ?

```using System; using System.Collections.Generic;
using System.Linq; using static System.Console; using System.Collections;

class Program {
static void Main(string[] args) {
WriteLine ("Multiply two consecutive prime numbers, add an even number," +
" see if the result is a prime number (up to a limit).");
int c, lim = 500; var pr = PG.Primes(lim * lim).ToList();
pr = pr.TakeWhile(x => x < lim).ToList();
var Lst = new[]{ Tuple.Create(2, 2), Tuple.Create(-20, 20) };
foreach (var pair in Lst) {
bool sho = pair.Item1 == pair.Item2;
for (int ofs = pair.Item1; ofs <= pair.Item2; ofs += ofs == -2 ? 4 : 2) {
c = 0; string s = ofs.ToString("+0;-#").Insert(1, " ");
for (int i = 0, j = 1, k; j < pr.Count; i = j++)
if (PG.isPr(k = pr[i] * pr[j] + ofs))
if (sho) WriteLine ("   {0,3} * {1,3} {2} = {3,-6}",
pr[i], pr[j], s, k, c++);
else c++;
WriteLine("{0,2} found under {1} for \" {2} \"", c, lim, s);
} WriteLine (); } } }

class PG { static bool[] flags; public static bool isPr(int x) {
if (x < 2) return false; return !flags[x]; }
public static IEnumerable<int> Primes(int lim) {
flags = new bool[lim + 1]; int j = 3;
for (int d = 8, sq = 9; sq <= lim; j += 2, sq += d += 8)
if (!flags[j]) { yield return j;
for (int k = sq, i=j<<1; k<=lim; k += i) flags[k] = true; }
for (; j <= lim; j += 2) if (!flags[j]) yield return j; } }
```
Output:
```Multiply two consecutive prime numbers, add an even number, see if the result is a prime number (up to a limit).
3 *   5 + 2 = 17
5 *   7 + 2 = 37
7 *  11 + 2 = 79
13 *  17 + 2 = 223
19 *  23 + 2 = 439
67 *  71 + 2 = 4759
149 * 151 + 2 = 22501
179 * 181 + 2 = 32401
229 * 233 + 2 = 53359
239 * 241 + 2 = 57601
241 * 251 + 2 = 60493
269 * 271 + 2 = 72901
277 * 281 + 2 = 77839
307 * 311 + 2 = 95479
313 * 317 + 2 = 99223
397 * 401 + 2 = 159199
401 * 409 + 2 = 164011
419 * 421 + 2 = 176401
439 * 443 + 2 = 194479
487 * 491 + 2 = 239119
20 found under 500 for " + 2 "

5 found under 500 for " - 20 "
26 found under 500 for " - 18 "
22 found under 500 for " - 16 "
10 found under 500 for " - 14 "
22 found under 500 for " - 12 "
21 found under 500 for " - 10 "
13 found under 500 for " - 8 "
32 found under 500 for " - 6 "
20 found under 500 for " - 4 "
5 found under 500 for " - 2 "
20 found under 500 for " + 2 "
9 found under 500 for " + 4 "
36 found under 500 for " + 6 "
18 found under 500 for " + 8 "
11 found under 500 for " + 10 "
27 found under 500 for " + 12 "
20 found under 500 for " + 14 "
8 found under 500 for " + 16 "
17 found under 500 for " + 18 "
25 found under 500 for " + 20 "```

## Delphi

Works with: Delphi version 6.0

```function IsPrime(N: int64): boolean;
{Fast, optimised prime test}
var I,Stop: int64;
begin
if (N = 2) or (N=3) then Result:=true
else if (n <= 1) or ((n mod 2) = 0) or ((n mod 3) = 0) then Result:= false
else
begin
I:=5;
Stop:=Trunc(sqrt(N+0.0));
Result:=False;
while I<=Stop do
begin
if ((N mod I) = 0) or ((N mod (I + 2)) = 0) then exit;
Inc(I,6);
end;
Result:=True;
end;
end;

function GetNextPrime(var Start: integer): integer;
{Get the next prime number after Start}
{Start is passed by "reference," so the
{original variable is incremented}
begin
repeat Inc(Start)
until IsPrime(Start);
Result:=Start;
end;

procedure ShowNeighborPrimes(Memo: TMemo);
var P1,P2,P3,Cnt: integer;
var S: string;
begin
Cnt:=0; P1:=1; P2:=1; S:='';
While P1< 500 do
begin
GetNextPrime(P2);
P3:=P1 * P2 + 2;
if IsPrime(P3) then
begin
Inc(Cnt);
S:=S+Format('%5D %4D %4D %6D',[Cnt,P1,P2,P3]);
S:=S+#\$0D#\$0A;
end;
P1:=P2;
end;
end;
```
Output:
```Count    P    Q   PQ+2
-----------------------
1    3    5     17
2    5    7     37
3    7   11     79
4   13   17    223
5   19   23    439
6   67   71   4759
7  149  151  22501
8  179  181  32401
9  229  233  53359
10  239  241  57601
11  241  251  60493
12  269  271  72901
13  277  281  77839
14  307  311  95479
15  313  317  99223
16  397  401 159199
17  401  409 164011
18  419  421 176401
19  439  443 194479
20  487  491 239119
```

## F#

This task uses Extensible Prime Generator (F#)

```// Nigel Galloway. April 13th., 2021
primes32()|>Seq.pairwise|>Seq.takeWhile(fun(n,_)->n<500)|>Seq.filter(fun(n,g)->isPrime(n*g+2))|>Seq.iter(fun(n,g)->printfn "%d*%d=%d" n g (n*g+2))
```
Output:
```3*5=17
5*7=37
7*11=79
13*17=223
19*23=439
67*71=4759
149*151=22501
179*181=32401
229*233=53359
239*241=57601
241*251=60493
269*271=72901
277*281=77839
307*311=95479
313*317=99223
397*401=159199
401*409=164011
419*421=176401
439*443=194479
487*491=239119
Real: 00:00:00.029
```

## Factor

Works with: Factor version 0.99 2021-02-05
```USING: formatting io kernel math math.primes ;

"p    q    p*q+2" print
2 3
[ over 500 < ] [
2dup * 2 + dup prime?
[ 3dup "%-4d %-4d %-6d\n" printf ] when
drop nip dup next-prime
] while 2drop
```
Output:
```p    q    p*q+2
3    5    17
5    7    37
7    11   79
13   17   223
19   23   439
67   71   4759
149  151  22501
179  181  32401
229  233  53359
239  241  57601
241  251  60493
269  271  72901
277  281  77839
307  311  95479
313  317  99223
397  401  159199
401  409  164011
419  421  176401
439  443  194479
487  491  239119
```

## Fermat

Translation of: PARI/GP
`for i = 1 to 95 do if Isprime(2+Prime(i)*Prime(i+1)) then !!Prime(i) fi od`

## FreeBASIC

```#include "isprime.bas"

dim as uinteger q

print "p             q             pq+2"
print "--------------------------------"
for p as uinteger = 2 to 499
if not isprime(p) then continue for
q = p + 1
while not isprime(q)
q+=1
wend
if not isprime( 2 + p*q ) then continue for
print p,q,2+p*q
next p```
Output:
```p             q             pq+2
--------------------------------
3             5             17
5             7             37
7             11            79
13            17            223
19            23            439
67            71            4759
149           151           22501
179           181           32401
229           233           53359
239           241           57601
241           251           60493
269           271           72901
277           281           77839
307           311           95479
313           317           99223
397           401           159199
401           409           164011
419           421           176401
439           443           194479
487           491           239119
```

## FutureBasic

```local fn IsPrime( n as NSUInteger ) as BOOL
BOOL       isPrime = YES
NSUInteger i

if n < 2        then exit fn = NO
if n = 2        then exit fn = YES
if n mod 2 == 0 then exit fn = NO
for i = 3 to int(n^.5) step 2
if n mod i == 0 then exit fn = NO
next
end fn = isPrime

local fn FindNeighborPrimes( searchLimit as long )
NSUInteger p, q

printf @"p       q       p*q+2"
printf @"----------------------"
for p = 2 to searchLimit
if ( fn IsPrime(p) == NO ) then continue
q = p + 1
while ( fn IsPrime(q) == NO )
q += 1
wend
if ( fn IsPrime( p * q + 2 ) == NO ) then continue
printf @"%lu\t\t%-6lu\t%-6lu", p, q, p * q + 2
next
end fn

fn FindNeighborPrimes( 499 )

HandleEvents```
Output:
```p       q       p*q+2
----------------------
3		5     	17
5		7     	37
7		11    	79
13		17    	223
19		23    	439
67		71    	4759
149		151   	22501
179		181   	32401
229		233   	53359
239		241   	57601
241		251   	60493
269		271   	72901
277		281   	77839
307		311   	95479
313		317   	99223
397		401   	159199
401		409   	164011
419		421   	176401
439		443   	194479
487		491   	239119

```

## Fōrmulæ

Fōrmulæ programs are not textual, visualization/edition of programs is done showing/manipulating structures but not text. Moreover, there can be multiple visual representations of the same program. Even though it is possible to have textual representation —i.e. XML, JSON— they are intended for storage and transfer purposes more than visualization and edition.

Programs in Fōrmulæ are created/edited online in its website.

In this page you can see and run the program(s) related to this task and their results. You can also change either the programs or the parameters they are called with, for experimentation, but remember that these programs were created with the main purpose of showing a clear solution of the task, and they generally lack any kind of validation.

Solution

## Go

Translation of: Wren
Library: Go-rcu
```package main

import (
"fmt"
"rcu"
)

func main() {
primes := rcu.Primes(504)
var nprimes []int
fmt.Println("Neighbour primes < 500:")
for i := 0; i < len(primes)-1; i++ {
p := primes[i]*primes[i+1] + 2
if rcu.IsPrime(p) {
nprimes = append(nprimes, primes[i])
}
}
rcu.PrintTable(nprimes, 10, 3, false)
fmt.Println("\nFound", len(nprimes), "such primes.")
}
```
Output:
```Neighbour primes < 500:
3   5   7  13  19  67 149 179 229 239
241 269 277 307 313 397 401 419 439 487

Found 20 such primes.
```

```import Data.List.Split ( divvy )

isPrime :: Int -> Bool
isPrime n
|n < 2 = False
|otherwise = null \$ filter (\i -> mod n i == 0 ) [2 .. root]
where
root :: Int
root = floor \$ sqrt \$ fromIntegral n

solution :: [Int]
solution = map head \$ filter (\li -> isPrime ((head li * last li) + 2 ))
\$ divvy 2 1 \$ filter isPrime [2..upTo]
where
upTo :: Int
upTo = head \$ take 1 \$ filter isPrime [500..]
```
Output:
```[3,5,7,13,19,67,149,179,229,239,241,269,277,307,313,397,401,419,439,487]
```

## J

```   (#~ 1 p: {:"1) 2 (, 2 + */)\ i.&.(p:inv) 500
3   5     17
5   7     37
7  11     79
13  17    223
19  23    439
67  71   4759
149 151  22501
179 181  32401
229 233  53359
239 241  57601
241 251  60493
269 271  72901
277 281  77839
307 311  95479
313 317  99223
397 401 159199
401 409 164011
419 421 176401
439 443 194479
487 491 239119
```

## jq

Works with: jq

Works with gojq, the Go implementation of jq

This entry uses `is_prime` as defined at Erdős-primes#jq.

```def next_prime:
if . == 2 then 3
else first(range(.+2; infinite; 2) | select(is_prime))
end;

# (not actually used)
def is_neighbour_prime:
is_prime and ((. * next_prime) + 2 | is_prime);

# The task, implemented using only `next_prime` for efficiency
{p: 2}
| while (.p < 500;
(.p|next_prime) as \$np
| .emit = false
| if (.p * \$np) + 2 | is_prime
then .emit = .p
else .
end
| .p = \$np )
| select(.emit).emit```
Output:
```3
5
7
13
19
67
149
179
229
239
241
269
277
307
313
397
401
419
439
487
```

## Julia

```using Primes

isneiprime(known) = isprime(known) && isprime(known * nextprime(known + 1) + 2)
println(filter(isneiprime, primes(500)))
```
Output:
`[3, 5, 7, 13, 19, 67, 149, 179, 229, 239, 241, 269, 277, 307, 313, 397, 401, 419, 439, 487]`

## Ksh

```#!/bin/ksh

# Find and show primes p such that p*q+2 is prime, where q is next prime after p and p<500

#	# Variables:
#
integer MAX_PRIME=500

typeset -a parr

#	# Functions:
#

#	# Function _isprime(n) return 1 for prime, 0 for not prime
#
function _isprime {
typeset _n ; integer _n=\$1
typeset _i ; integer _i

(( _n < 2 )) && return 0
for (( _i=2 ; _i*_i<=_n ; _i++ )); do
(( ! ( _n % _i ) )) && return 0
done
return 1
}

#	# Function _neighbourprime(n) return p*q+2 if prime; 0 if not
#
function _neighbourprime {
typeset _indx ; integer _indx=\$1
typeset _arr ; nameref _arr="\$2"
typeset _neighbor

(( _neighbor = _arr[_indx] * _arr[_indx+1] + 2 ))
_isprime \${_neighbor}
(( \$? )) && echo \${_neighbor} && return
echo 0
}

######
# main #
######

for ((i=2; i<MAX_PRIME; i++)); do
_isprime \${i} ; (( \$? )) && parr+=( \${i} )
done

printf "%3s %3s %6s\n" p q p*q+2
printf "%3s %3s %6s\n" --- --- -----
for ((i=0; i<\$((\${#parr[*]}-1)); i++)); do
np=\$(_neighbourprime \${i} parr)
(( np > 0 )) && printf "%3d %3d %6d\n" \${parr[i]} \${parr[i+1]} \${np}
done
```
Output:
```
p   q  p*q+2

--- ---  -----

3   5     17
5   7     37
7  11     79
13  17    223
19  23    439
67  71   4759

149 151  22501
179 181  32401
229 233  53359
239 241  57601
241 251  60493
269 271  72901
277 281  77839
307 311  95479
313 317  99223
397 401 159199
401 409 164011
419 421 176401
439 443 194479
487 491 239119

```

## Mathematica/Wolfram Language

```p = Prime@Range@PrimePi[499];
Select[p, PrimeQ[# NextPrime[#] + 2] &]
```
Output:
`{3, 5, 7, 13, 19, 67, 149, 179, 229, 239, 241, 269, 277, 307, 313, 397, 401, 419, 439, 487}`

## Nim

```import strformat, sugar

const
Max1 = 499        # Maximum for first prime.
Max2 = 251_000    # Maximum for sieve (in fact 250_999 = 499 * 503 + 2).

# Sieve of Erathosthenes: false (default) is composite.
var composite: array[3..Max2, bool]   # Ignore 2 as 2 * 3 + 8 is not prime.
var n = 3
while true:
let n2 = n * n
if n2 > Max2: break
if not composite[n]:
for k in countup(n2, Max2, 2 * n):
composite[k] = true
inc n, 2

template isPrime(n: int): bool = not composite[n]

let primes = collect(newSeq):
for n in countup(3, Max2, 2):
if n.isPrime: n

var p = primes[0]
var i = 0
while p <= Max1:
inc i
let q = primes[i]
if (p * q + 2).isPrime:
echo &"{p:3} {q:3} {p*q+2:6}"
p = q
```
Output:
```  3   5     17
5   7     37
7  11     79
13  17    223
19  23    439
67  71   4759
149 151  22501
179 181  32401
229 233  53359
239 241  57601
241 251  60493
269 271  72901
277 281  77839
307 311  95479
313 317  99223
397 401 159199
401 409 164011
419 421 176401
439 443 194479
487 491 239119```

## PARI/GP

Cheats a little in the sense that it requires knowing the 95th prime is 499 beforehand.

`for(i=1, 95, if(isprime(2+prime(i)*prime(i+1)),print(prime(i))))`

## Perl

Library: ntheory
```use strict;
use warnings;
use ntheory <next_prime is_prime>;

my \$p = 2;
do {
my \$q = next_prime(\$p);
printf "%3d%5d%8d\n", \$p, \$q, \$p*\$q+2 if is_prime \$p*\$q+2;
\$p = \$q;
} until \$p >= 500;
```
Output:
```  3    5      17
5    7      37
7   11      79
13   17     223
19   23     439
67   71    4759
149  151   22501
179  181   32401
229  233   53359
239  241   57601
241  251   60493
269  271   72901
277  281   77839
307  311   95479
313  317   99223
397  401  159199
401  409  164011
419  421  176401
439  443  194479
487  491  239119```

## Phix

```function np(integer p) return is_prime(get_prime(p)*get_prime(p+1)+2) end function
constant N = length(get_primes_le(500))
sequence res = apply(apply(filter(tagset(N),np),get_prime),sprint)
printf(1,"Found %d such primes: %s\n",{length(res),join(shorten(res,"",5),", ")})
```
Output:
```Found 20 such primes: 3, 5, 7, 13, 19, ..., 397, 401, 419, 439, 487
```

## PL/0

Formatted output isn't PL/0's forté, so this sample just shows each p1 of the p1, p2 neighbours.
This is almost identical to the PL/0 sample in the Special Neighbor primes task

```var   n, p1, p2, prime;
procedure isnprime;
var p;
begin
prime := 1;
if n < 2 then prime := 0;
if n > 2 then begin
prime := 0;
if odd( n ) then prime := 1;
p := 3;
while p * p <= n * prime do begin
if n - ( ( n / p ) * p ) = 0 then prime := 0;
p := p + 2;
end
end
end;
begin
p1 := 3;
p2 := 5;
while p2 < 500 do begin
n := ( p1 * p2 ) + 2;
call isnprime;
if prime = 1 then ! p1;
n  := p2 + 2;
call isnprime;
while prime = 0 do begin
n := n + 2;
call isnprime;
end;
p1 := p2;
p2 := n;
end
end.
```
Output:
```          3
5
7
13
19
67
149
179
229
239
241
269
277
307
313
397
401
419
439
487
```

## Python

```#!/usr/bin/python

def isPrime(n):
for i in range(2, int(n**0.5) + 1):
if n % i == 0:
return False
return True

if __name__ == '__main__':
print("p        q       pq+2")
print("-----------------------")
for p in range(2, 499):
if not isPrime(p):
continue
q = p + 1
while not isPrime(q):
q += 1
if not isPrime(2 + p*q):
continue
print(p, "\t", q, "\t", 2+p*q)
```
Output:
```p        q       pq+2
-----------------------
3 	 5 	 17
5 	 7 	 37
7 	 11 	 79
13 	 17 	 223
19 	 23 	 439
67 	 71 	 4759
149 	 151 	 22501
179 	 181 	 32401
229 	 233 	 53359
239 	 241 	 57601
241 	 251 	 60493
269 	 271 	 72901
277 	 281 	 77839
307 	 311 	 95479
313 	 317 	 99223
397 	 401 	 159199
401 	 409 	 164011
419 	 421 	 176401
439 	 443 	 194479
487 	 491 	 239119```

## Raku

```my @primes = grep &is-prime, ^Inf;
my \$last_p = @primes.first: :k, * >= 500;
my \$last_q = \$last_p + 1;

my @cousins = @primes.head( \$last_q )
.rotor( 2 => -1 )
.map(-> (\p, \q) { p, q, p*q+2 } )
.grep( *.[2].is-prime );

say .fmt('%6d') for @cousins;
```
Output:
```     3      5     17
5      7     37
7     11     79
13     17    223
19     23    439
67     71   4759
149    151  22501
179    181  32401
229    233  53359
239    241  57601
241    251  60493
269    271  72901
277    281  77839
307    311  95479
313    317  99223
397    401 159199
401    409 164011
419    421 176401
439    443 194479
487    491 239119
```

## REXX

Neighbor primes can also be spelled neighbour primes.

```/*REXX program finds neighbor primes: P, Q, P*Q+2 are primes, and  P < some specified N.*/
parse arg hi cols .                              /*obtain optional argument from the CL.*/
if   hi=='' |   hi==","  then   hi=  500         /*Not specified?  Then use the default.*/
if cols=='' | cols==","  then cols=   10         /* "      "         "   "   "     "    */
call genP hi+50                                  /*build semaphore array for low primes.*/
do p=1  while @.p<hi
end  /*p*/;           lim= p-1;   q= p+1    /*set LIM to prime for P; calc. 2nd HI.*/
call genP @.p * @.q  +  2                        /*build semaphore array for high primes*/
w= 10                                            /*width of a number in any column.     */
@neig= ' neighbor primes:  p, q, p*q+2  are primes,  and p  < '  commas(hi)
if cols>0 then say ' index │'center(@neig,   1 + cols*(w+1)     )
if cols>0 then say '───────┼'center(""   ,   1 + cols*(w+1), '─')
Nprimes= 0;                idx= 1                /*initialize # neighbor primes & index.*/
\$=                                               /*a list of  neighbor  primes (so far).*/
do j=1  to  lim;      jp= j+1;   q= @.jp    /*look for neighbor primes within range*/
x= @.j * q  +  2;     if \!.x  then iterate /*is X also a prime?  No, then skip it.*/
Nprimes= Nprimes + 1                        /*bump the number of  neighbor primes. */
if cols==0            then iterate          /*Build the list  (to be shown later)? */
\$= \$ right( commas(@.j), w)                 /*add neighbor prime ──► the  \$  list. */
if Nprimes//cols\==0  then iterate          /*have we populated a line of output?  */
say center(idx, 7)'│'  substr(\$, 2);   \$=   /*display what we have so far  (cols). */
idx= idx + cols                             /*bump the  index  count for the output*/
end   /*j*/

if \$\==''  then say center(idx, 7)"│"  substr(\$, 2)  /*possible display residual output.*/
if cols>0 then say '───────┴'center(""                         ,  1 + cols*(w+1), '─')
say
say 'Found '       commas(Nprimes)      @neig
exit 0                                           /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
commas: parse arg ?;  do jc=length(?)-3  to 1  by -3; ?=insert(',', ?, jc); end;  return ?
/*──────────────────────────────────────────────────────────────────────────────────────*/
genP: !.= 0;            parse arg limit          /*placeholders for primes (semaphores).*/
@.1=2;  @.2=3;  @.3=5;  @.4=7;  @.5=11     /*define some low primes.              */
!.2=1;  !.3=1;  !.5=1;  !.7=1;  !.11=1     /*   "     "   "    "     flags.       */
#=5;     s.#= @.# **2    /*number of primes so far;     prime². */
/* [↓]  generate more  primes  ≤  high.*/
do j=@.#+2  by 2  to limit               /*find odd primes from here on.        */
parse var j '' -1 _;       if    _==5  then iterate  /*J ÷ by 5?  (right digit).*/
if j//3==0  then iterate;  if j//7==0  then iterate  /*" "  " 3?     J ÷ by 7?  */
do k=5  while s.k<=j              /* [↓]  divide by the known odd primes.*/
if j // @.k == 0  then iterate j  /*Is  J ÷ X?  Then not prime.     ___  */
end   /*k*/                       /* [↑]  only process numbers  ≤  √ J   */
#= #+1;    @.#= j;    s.#= j*j;   !.j= 1 /*bump # of Ps; assign next P;  P²; P# */
end          /*j*/;   return
```
output   when using the default inputs:
``` index │                           neighbor primes:  p, q, p*q+2  are primes,  and p  <  500
───────┼───────────────────────────────────────────────────────────────────────────────────────────────────────────────
1   │          3          5          7         13         19         67        149        179        229        239
11   │        241        269        277        307        313        397        401        419        439        487
───────┴───────────────────────────────────────────────────────────────────────────────────────────────────────────────

Found  20  neighbor primes:  p, q, p*q+2  are primes,  and p  <  500
```

## Ring

```load "stdlib.ring"
see "working..." + nl
see "Neighbour primes are:" + nl
see "p q p*q+2" + nl

row = 0
num = 0
pr = 0
limit = 100
Primes = []

while true
pr = pr + 1
if isprime(pr)
num = num + 1
if num = limit
exit
ok
ok
end

for n = 1 to limit-1
prim = Primes[n]*Primes[n+1]+2
if isprime(prim)
row = row + 1
see "" + Primes[n] + " " + Primes[n+1] + " " + prim + nl
ok
next

see "Found " + row + " neighbour primes" + nl
see "done..." + nl```
Output:
```working...
Neighbour primes are:
p q p*q+2
3 5 17
5 7 37
7 11 79
13 17 223
19 23 439
67 71 4759
149 151 22501
179 181 32401
229 233 53359
239 241 57601
241 251 60493
269 271 72901
277 281 77839
307 311 95479
313 317 99223
397 401 159199
401 409 164011
419 421 176401
439 443 194479
487 491 239119
Found 20 neighbour primes
done...
```

## Rust

```fn main() {
let mut primes_first : Vec<u64> = Vec::new( ) ;
primal::Primes::all( ).take_while( | n | *n < 500 ).for_each( | num |
primes_first.push( num as u64 ) ) ;
let mut current : u64 = *primes_first.iter( ).last( ).unwrap( ) + 1 ;
while ! primal::is_prime( current ) {
current += 1 ;
}
primes_first.push( current ) ;
let len = primes_first.len( ) ;
let mut primes_searched : Vec<u64> = Vec::new( ) ;
for i in 0..len - 2 {
if primal::is_prime( primes_first[ i ] * primes_first[ i + 1 ] + 2 ) {
let num = primes_first[ i ] ;
primes_searched.push( num ) ;
}
}
println!("{:?}" , primes_searched ) ;
}
```
Output:
```[3, 5, 7, 13, 19, 67, 149, 179, 229, 239, 241, 269, 277, 307, 313, 397, 401, 419, 439, 487]
```

## RPL

Works with: HP version 49
```≪ → max
≪ { } 2
WHILE DUP max < REPEAT
DUP NEXTPRIME
IF DUP2 * 2 + ISPRIME? THEN UNROT + SWAP ELSE NIP END
END DROP
≫ ≫ 'NEIGHB' STO
```
```500 NEIGHB
```
Output:
```1: {3 5 7 13 19 67 149 179 229 239 241 269 277 307 313 397 401 419 439 487}
```

## Ruby

```require 'prime'

p Prime.each(500).each_cons(2).select{|p, q| (p*q+2).prime? }
```
Output:
```[[3, 5], [5, 7], [7, 11], [13, 17], [19, 23], [67, 71], [149, 151], [179, 181], [229, 233], [239, 241], [241, 251], [269, 271], [277, 281], [307, 311], [313, 317], [397, 401], [401, 409], [419, 421], [439, 443], [487, 491]]
```

## Sidef

```500.primes.grep {|p| p * p.next_prime + 2 -> is_prime }.say
```
Output:
```[3, 5, 7, 13, 19, 67, 149, 179, 229, 239, 241, 269, 277, 307, 313, 397, 401, 419, 439, 487]
```

## Wren

Library: Wren-math
Library: Wren-fmt
```import "./math" for Int
import "./fmt" for Fmt

var primes = Int.primeSieve(504)
var nprimes = []
System.print("Neighbour primes < 500:")
for (i in 0...primes.count-1) {
var p = primes[i] * primes[i+1] + 2
}
Fmt.tprint("\$3d", nprimes, 10)
System.print("\nFound %(nprimes.count) such primes.")
```
Output:
```Neighbour primes < 500:
3   5   7  13  19  67 149 179 229 239
241 269 277 307 313 397 401 419 439 487

Found 20 such primes.
```

## XPL0

```func IsPrime(N);        \Return 'true' if N is a prime number
int  N, I;
[if N <= 1 then return false;
for I:= 2 to sqrt(N) do
if rem(N/I) = 0 then return false;
return true;
];

int Count, P, Q;
[Count:= 0;
P:= 2;  Q:= 3;
repeat  if IsPrime(Q) then
[if IsPrime(P*Q+2) then
[IntOut(0, P);
ChOut(0, ^ );
Count:= Count+1;
];
P:= Q;
];
Q:= Q+2;
until   P >= 500;
CrLf(0);
IntOut(0, Count);
Text(0, " neighbour primes found below 500.
");
]```
Output:
```3 5 7 13 19 67 149 179 229 239 241 269 277 307 313 397 401 419 439 487
20 neighbour primes found below 500.
```