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Chernick's Carmichael numbers

From Rosetta Code
Task
Chernick's Carmichael numbers
You are encouraged to solve this task according to the task description, using any language you may know.

In 1939, Jack Chernick proved that, for n ≥ 3 and m ≥ 1:

   U(n, m) = (6m + 1) * (12m + 1) * Product_{i=1..n-2} (2^i * 9m + 1)

is a Carmichael number if all the factors are primes and, for n > 4, m is a multiple of 2^(n-4).


Example
   U(3, m) = (6m + 1) * (12m + 1) * (18m + 1)
   U(4, m) = U(3, m) * (2^2 * 9m + 1)
   U(5, m) = U(4, m) * (2^3 * 9m + 1)
   ...
   U(n, m) = U(n-1, m) * (2^(n-2) * 9m + 1)
  • The smallest Chernick's Carmichael number with 3 prime factors, is: U(3, 1) = 1729.
  • The smallest Chernick's Carmichael number with 4 prime factors, is: U(4, 1) = 63973.
  • The smallest Chernick's Carmichael number with 5 prime factors, is: U(5, 380) = 26641259752490421121.


For n = 5, the smallest number m that satisfy Chernick's conditions, is m = 380, therefore U(5, 380) is the smallest Chernick's Carmichael number with 5 prime factors.

U(5, 380) is a Chernick's Carmichael number because m = 380 is a multiple of 2^(n-4), where n = 5, and the factors { (6*380 + 1), (12*380 + 1), (18*380 + 1), (36*380 + 1), (72*380 + 1) } are all prime numbers.


Task

For n ≥ 3, let a(n) be the smallest Chernick's Carmichael number with n prime factors.

  • Compute a(n) for n = 3..9.
  • Optional: find a(10).


Note: it's perfectly acceptable to show the terms in factorized form:

 a(3) = 7 * 13 * 19
 a(4) = 7 * 13 * 19 * 37
 a(5) = 2281 * 4561 * 6841 * 13681 * 27361
 ...


See also


Related tasks



C[edit]

Library: GMP
#include <stdio.h>
#include <stdlib.h>
#include <gmp.h>
 
typedef unsigned long long int u64;
 
#define TRUE 1
#define FALSE 0
 
int primality_pretest(u64 k) {
if (!(k % 3) || !(k % 5) || !(k % 7) || !(k % 11) || !(k % 13) || !(k % 17) || !(k % 19) || !(k % 23)) return (k <= 23);
return TRUE;
}
 
int probprime(u64 k, mpz_t n) {
mpz_set_ui(n, k);
return mpz_probab_prime_p(n, 0);
}
 
int is_chernick(int n, u64 m, mpz_t z) {
u64 t = 9 * m;
if (primality_pretest(6 * m + 1) == FALSE) return FALSE;
if (primality_pretest(12 * m + 1) == FALSE) return FALSE;
for (int i = 1; i <= n - 2; i++) if (primality_pretest((t << i) + 1) == FALSE) return FALSE;
if (probprime(6 * m + 1, z) == FALSE) return FALSE;
if (probprime(12 * m + 1, z) == FALSE) return FALSE;
for (int i = 1; i <= n - 2; i++) if (probprime((t << i) + 1, z) == FALSE) return FALSE;
return TRUE;
}
 
int main(int argc, char const *argv[]) {
mpz_t z;
mpz_inits(z, NULL);
 
for (int n = 3; n <= 10; n ++) {
u64 multiplier = (n > 4) ? (1 << (n - 4)) : 1;
 
if (n > 5) multiplier *= 5;
 
for (u64 k = 1; ; k++) {
u64 m = k * multiplier;
 
if (is_chernick(n, m, z) == TRUE) {
printf("a(%d) has m = %llu\n", n, m);
break;
}
}
}
 
return 0;
}
Output:
a(3) has m = 1
a(4) has m = 1
a(5) has m = 380
a(6) has m = 380
a(7) has m = 780320
a(8) has m = 950560
a(9) has m = 950560
a(10) has m = 3208386195840

C++[edit]

Library: GMP
#include <gmp.h>
#include <iostream>
 
using namespace std;
 
typedef unsigned long long int u64;
 
bool primality_pretest(u64 k) { // for k > 23
 
if (!(k % 3) || !(k % 5) || !(k % 7) || !(k % 11) ||
!(k % 13) || !(k % 17) || !(k % 19) || !(k % 23)
) {
return (k <= 23);
}
 
return true;
}
 
bool probprime(u64 k, mpz_t n) {
mpz_set_ui(n, k);
return mpz_probab_prime_p(n, 0);
}
 
bool is_chernick(int n, u64 m, mpz_t z) {
 
if (!primality_pretest(6 * m + 1)) {
return false;
}
 
if (!primality_pretest(12 * m + 1)) {
return false;
}
 
u64 t = 9 * m;
 
for (int i = 1; i <= n - 2; i++) {
if (!primality_pretest((t << i) + 1)) {
return false;
}
}
 
if (!probprime(6 * m + 1, z)) {
return false;
}
 
if (!probprime(12 * m + 1, z)) {
return false;
}
 
for (int i = 1; i <= n - 2; i++) {
if (!probprime((t << i) + 1, z)) {
return false;
}
}
 
return true;
}
 
int main() {
 
mpz_t z;
mpz_inits(z, NULL);
 
for (int n = 3; n <= 10; n++) {
 
// `m` is a multiple of 2^(n-4), for n > 4
u64 multiplier = (n > 4) ? (1 << (n - 4)) : 1;
 
// For n > 5, m is also a multiple of 5
if (n > 5) {
multiplier *= 5;
}
 
for (u64 k = 1; ; k++) {
 
u64 m = k * multiplier;
 
if (is_chernick(n, m, z)) {
cout << "a(" << n << ") has m = " << m << endl;
break;
}
}
}
 
return 0;
}
Output:
a(3) has m = 1
a(4) has m = 1
a(5) has m = 380
a(6) has m = 380
a(7) has m = 780320
a(8) has m = 950560
a(9) has m = 950560
a(10) has m = 3208386195840

(takes ~3.5 minutes)

F#[edit]

This task uses Extensible Prime Generator (F#)

 
// Generate Chernick's Carmichael numbers. Nigel Galloway: June 1st., 2019
let fMk m k=isPrime(6*m+1) && isPrime(12*m+1) && [1..k-2]|>List.forall(fun n->isPrime(9*(pown 2 n)*m+1))
let fX k=Seq.initInfinite(fun n->(n+1)*(pown 2 (k-4))) |> Seq.filter(fun n->fMk n k )
let cherCar k=let m=Seq.head(fX k) in printfn "m=%d primes -> %A " m ([6*m+1;12*m+1]@List.init(k-2)(fun n->9*(pown 2 (n+1))*m+1))
[4..9] |> Seq.iter cherCar
 
Output:
cherCar(4): m=1 primes -> [7; 13; 19; 37] 
cherCar(5): m=380 primes -> [2281; 4561; 6841; 13681; 27361] 
cherCar(6): m=380 primes -> [2281; 4561; 6841; 13681; 27361; 54721] 
cherCar(7): m=780320 primes -> [4681921; 9363841; 14045761; 28091521; 56183041; 112366081; 224732161] 
cherCar(8): m=950560 primes -> [5703361; 11406721; 17110081; 34220161; 68440321; 136880641; 273761281; 547522561] 
cherCar(9): m=950560 primes -> [5703361; 11406721; 17110081; 34220161; 68440321; 136880641; 273761281; 547522561; 1095045121] 

Go[edit]

Basic only[edit]

package main
 
import (
"fmt"
"math/big"
)
 
var (
zero = new(big.Int)
prod = new(big.Int)
fact = new(big.Int)
)
 
func ccFactors(n, m uint64) (*big.Int, bool) {
prod.SetUint64(6*m + 1)
if !prod.ProbablyPrime(0) {
return zero, false
}
fact.SetUint64(12*m + 1)
if !fact.ProbablyPrime(0) { // 100% accurate up to 2 ^ 64
return zero, false
}
prod.Mul(prod, fact)
for i := uint64(1); i <= n-2; i++ {
fact.SetUint64((1<<i)*9*m + 1)
if !fact.ProbablyPrime(0) {
return zero, false
}
prod.Mul(prod, fact)
}
return prod, true
}
 
func ccNumbers(start, end uint64) {
for n := start; n <= end; n++ {
m := uint64(1)
if n > 4 {
m = 1 << (n - 4)
}
for {
num, ok := ccFactors(n, m)
if ok {
fmt.Printf("a(%d) = %d\n", n, num)
break
}
if n <= 4 {
m++
} else {
m += 1 << (n - 4)
}
}
}
}
 
func main() {
ccNumbers(3, 9)
}
Output:
a(3) = 1729
a(4) = 63973
a(5) = 26641259752490421121
a(6) = 1457836374916028334162241
a(7) = 24541683183872873851606952966798288052977151461406721
a(8) = 53487697914261966820654105730041031613370337776541835775672321
a(9) = 58571442634534443082821160508299574798027946748324125518533225605795841

Basic plus optional[edit]


To reach a(10) in a reasonable time, a much more efficient approach is needed.

The following version takes account of the optimizations referred to in the Talk page and previewed in the C++ entry above.

It also uses a wrapper for the C library, GMP, which despite the overhead of cgo is still much faster than Go's native big.Int library.

The resulting executable is several hundred times faster than before and, even on my modest Celeron @1.6GHZ, reaches a(9) in under 10ms and a(10) in about 22 minutes.

package main
 
import (
"fmt"
big "github.com/ncw/gmp"
)
 
const (
min = 3
max = 10
)
 
var (
prod = new(big.Int)
fact = new(big.Int)
factors = [max]uint64{}
bigFactors = [max]*big.Int{}
)
 
func init() {
for i := 0; i < max; i++ {
bigFactors[i] = big.NewInt(0)
}
}
 
func isPrimePretest(k uint64) bool {
if k%3 == 0 || k%5 == 0 || k%7 == 0 || k%11 == 0 ||
k%13 == 0 || k%17 == 0 || k%19 == 0 || k%23 == 0 {
return k <= 23
}
return true
}
 
func ccFactors(n, m uint64) bool {
if !isPrimePretest(6*m + 1) {
return false
}
if !isPrimePretest(12*m + 1) {
return false
}
factors[0] = 6*m + 1
factors[1] = 12*m + 1
t := 9 * m
for i := uint64(1); i <= n-2; i++ {
tt := (t << i) + 1
if !isPrimePretest(tt) {
return false
}
factors[i+1] = tt
}
 
for i := 0; i < int(n); i++ {
fact.SetUint64(factors[i])
if !fact.ProbablyPrime(0) {
return false
}
bigFactors[i].Set(fact)
}
return true
}
 
func prodFactors(n uint64) *big.Int {
prod.Set(bigFactors[0])
for i := 1; i < int(n); i++ {
prod.Mul(prod, bigFactors[i])
}
return prod
}
 
func ccNumbers(start, end uint64) {
for n := start; n <= end; n++ {
mult := uint64(1)
if n > 4 {
mult = 1 << (n - 4)
}
if n > 5 {
mult *= 5
}
m := mult
for {
if ccFactors(n, m) {
num := prodFactors(n)
fmt.Printf("a(%d) = %d\n", n, num)
fmt.Printf("m(%d) = %d\n", n, m)
fmt.Println("Factors:", factors[:n], "\n")
break
}
m += mult
}
}
}
 
func main() {
ccNumbers(min, max)
}
Output:
a(3) = 1729
m(3) = 1
Factors: [7 13 19] 

a(4) = 63973
m(4) = 1
Factors: [7 13 19 37] 

a(5) = 26641259752490421121
m(5) = 380
Factors: [2281 4561 6841 13681 27361] 

a(6) = 1457836374916028334162241
m(6) = 380
Factors: [2281 4561 6841 13681 27361 54721] 

a(7) = 24541683183872873851606952966798288052977151461406721
m(7) = 780320
Factors: [4681921 9363841 14045761 28091521 56183041 112366081 224732161] 

a(8) = 53487697914261966820654105730041031613370337776541835775672321
m(8) = 950560
Factors: [5703361 11406721 17110081 34220161 68440321 136880641 273761281 547522561] 

a(9) = 58571442634534443082821160508299574798027946748324125518533225605795841
m(9) = 950560
Factors: [5703361 11406721 17110081 34220161 68440321 136880641 273761281 547522561 1095045121] 

a(10) = 24616075028246330441656912428380582403261346369700917629170235674289719437963233744091978433592331048416482649086961226304033068172880278517841921
m(10) = 3208386195840
Factors: [19250317175041 38500634350081 57750951525121 115501903050241 231003806100481 462007612200961 924015224401921 1848030448803841 3696060897607681 7392121795215361] 

Java[edit]

 
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.List;
 
public class ChernicksCarmichaelNumbers {
 
public static void main(String[] args) {
for ( long n = 3 ; n < 10 ; n++ ) {
long m = 0;
boolean foundComposite = true;
List<Long> factors = null;
while ( foundComposite ) {
m += (n <= 4 ? 1 : (long) Math.pow(2, n-4) * 5);
factors = U(n, m);
foundComposite = false;
for ( long factor : factors ) {
if ( ! isPrime(factor) ) {
foundComposite = true;
break;
}
}
}
System.out.printf("U(%d, %d) = %s = %s %n", n, m, display(factors), multiply(factors));
}
}
 
private static String display(List<Long> factors) {
return factors.toString().replace("[", "").replace("]", "").replaceAll(", ", " * ");
}
 
private static BigInteger multiply(List<Long> factors) {
BigInteger result = BigInteger.ONE;
for ( long factor : factors ) {
result = result.multiply(BigInteger.valueOf(factor));
}
return result;
}
 
private static List<Long> U(long n, long m) {
List<Long> factors = new ArrayList<>();
factors.add(6*m + 1);
factors.add(12*m + 1);
for ( int i = 1 ; i <= n-2 ; i++ ) {
factors.add(((long)Math.pow(2, i)) * 9 * m + 1);
}
return factors;
}
 
private static final int MAX = 100_000;
private static final boolean[] primes = new boolean[MAX];
private static boolean SIEVE_COMPLETE = false;
 
private static final boolean isPrimeTrivial(long test) {
if ( ! SIEVE_COMPLETE ) {
sieve();
SIEVE_COMPLETE = true;
}
return primes[(int) test];
}
 
private static final void sieve() {
// primes
for ( int i = 2 ; i < MAX ; i++ ) {
primes[i] = true;
}
for ( int i = 2 ; i < MAX ; i++ ) {
if ( primes[i] ) {
for ( int j = 2*i ; j < MAX ; j += i ) {
primes[j] = false;
}
}
}
}
 
// See http://primes.utm.edu/glossary/page.php?sort=StrongPRP
public static final boolean isPrime(long testValue) {
if ( testValue == 2 ) return true;
if ( testValue % 2 == 0 ) return false;
if ( testValue <= MAX ) return isPrimeTrivial(testValue);
long d = testValue-1;
int s = 0;
while ( d % 2 == 0 ) {
s += 1;
d /= 2;
}
if ( testValue < 1373565L ) {
if ( ! aSrp(2, s, d, testValue) ) {
return false;
}
if ( ! aSrp(3, s, d, testValue) ) {
return false;
}
return true;
}
if ( testValue < 4759123141L ) {
if ( ! aSrp(2, s, d, testValue) ) {
return false;
}
if ( ! aSrp(7, s, d, testValue) ) {
return false;
}
if ( ! aSrp(61, s, d, testValue) ) {
return false;
}
return true;
}
if ( testValue < 10000000000000000L ) {
if ( ! aSrp(3, s, d, testValue) ) {
return false;
}
if ( ! aSrp(24251, s, d, testValue) ) {
return false;
}
return true;
}
// Try 5 "random" primes
if ( ! aSrp(37, s, d, testValue) ) {
return false;
}
if ( ! aSrp(47, s, d, testValue) ) {
return false;
}
if ( ! aSrp(61, s, d, testValue) ) {
return false;
}
if ( ! aSrp(73, s, d, testValue) ) {
return false;
}
if ( ! aSrp(83, s, d, testValue) ) {
return false;
}
//throw new RuntimeException("ERROR isPrime: Value too large = "+testValue);
return true;
}
 
private static final boolean aSrp(int a, int s, long d, long n) {
long modPow = modPow(a, d, n);
//System.out.println("a = "+a+", s = "+s+", d = "+d+", n = "+n+", modpow = "+modPow);
if ( modPow == 1 ) {
return true;
}
int twoExpR = 1;
for ( int r = 0 ; r < s ; r++ ) {
if ( modPow(modPow, twoExpR, n) == n-1 ) {
return true;
}
twoExpR *= 2;
}
return false;
}
 
private static final long SQRT = (long) Math.sqrt(Long.MAX_VALUE);
 
public static final long modPow(long base, long exponent, long modulus) {
long result = 1;
while ( exponent > 0 ) {
if ( exponent % 2 == 1 ) {
if ( result > SQRT || base > SQRT ) {
result = multiply(result, base, modulus);
}
else {
result = (result * base) % modulus;
}
}
exponent >>= 1;
if ( base > SQRT ) {
base = multiply(base, base, modulus);
}
else {
base = (base * base) % modulus;
}
}
return result;
}
 
 
// Result is a*b % mod, without overflow.
public static final long multiply(long a, long b, long modulus) {
long x = 0;
long y = a % modulus;
long t;
while ( b > 0 ) {
if ( b % 2 == 1 ) {
t = x + y;
x = (t > modulus ? t-modulus : t);
}
t = y << 1;
y = (t > modulus ? t-modulus : t);
b >>= 1;
}
return x % modulus;
}
 
}
 
Output:
U(3, 1) = 7 * 13 * 19 = 1729 
U(4, 1) = 7 * 13 * 19 * 37 = 63973 
U(5, 380) = 2281 * 4561 * 6841 * 13681 * 27361 = 26641259752490421121 
U(6, 380) = 2281 * 4561 * 6841 * 13681 * 27361 * 54721 = 1457836374916028334162241 
U(7, 780320) = 4681921 * 9363841 * 14045761 * 28091521 * 56183041 * 112366081 * 224732161 = 24541683183872873851606952966798288052977151461406721 
U(8, 950560) = 5703361 * 11406721 * 17110081 * 34220161 * 68440321 * 136880641 * 273761281 * 547522561 = 53487697914261966820654105730041031613370337776541835775672321 
U(9, 950560) = 5703361 * 11406721 * 17110081 * 34220161 * 68440321 * 136880641 * 273761281 * 547522561 * 1095045121 = 58571442634534443082821160508299574798027946748324125518533225605795841 

Julia[edit]

using Primes
 
function trial_pretest(k::UInt64)
 
if ((k % 3)==0 || (k % 5)==0 || (k % 7)==0 || (k % 11)==0 ||
(k % 13)==0 || (k % 17)==0 || (k % 19)==0 || (k % 23)==0)
return (k <= 23)
end
 
return true
end
 
function gcd_pretest(k::UInt64)
 
if (k <= 107)
return true
end
 
gcd(29*31*37*41*43*47*53*59*61*67, k) == 1 &&
gcd(71*73*79*83*89*97*101*103*107, k) == 1
end
 
function is_chernick(n::Int64, m::UInt64)
 
t = 9*m
 
if (!trial_pretest(6*m + 1))
return false
end
 
if (!trial_pretest(12*m + 1))
return false
end
 
for i in 1:n-2
if (!trial_pretest((t << i) + 1))
return false
end
end
 
if (!gcd_pretest(6*m + 1))
return false
end
 
if (!gcd_pretest(12*m + 1))
return false
end
 
for i in 1:n-2
if (!gcd_pretest((t << i) + 1))
return false
end
end
 
if (!isprime(6*m + 1))
return false
end
 
if (!isprime(12*m + 1))
return false
end
 
for i in 1:n-2
if (!isprime((t << i) + 1))
return false
end
end
 
return true
end
 
function chernick_carmichael(n::Int64, m::UInt64)
prod = big(1)
 
prod *= 6*m + 1
prod *= 12*m + 1
 
for i in 1:n-2
prod *= ((big(9)*m)<<i) + 1
end
 
prod
end
 
function cc_numbers(from, to)
 
for n in from:to
 
multiplier = 1
 
if (n > 4) multiplier = 1 << (n-4) end
if (n > 5) multiplier *= 5 end
 
m = UInt64(multiplier)
 
while true
 
if (is_chernick(n, m))
println("a(", n, ") = ", chernick_carmichael(n, m))
break
end
 
m += multiplier
end
end
end
 
cc_numbers(3, 10)
Output:
a(3) = 1729
a(4) = 63973
a(5) = 26641259752490421121
a(6) = 1457836374916028334162241
a(7) = 24541683183872873851606952966798288052977151461406721
a(8) = 53487697914261966820654105730041031613370337776541835775672321
a(9) = 58571442634534443082821160508299574798027946748324125518533225605795841
a(10) = 24616075028246330441656912428380582403261346369700917629170235674289719437963233744091978433592331048416482649086961226304033068172880278517841921

(takes ~6.5 minutes)

Mathematica / Wolfram Language[edit]

ClearAll[PrimeFactorCounts, U]
PrimeFactorCounts[n_Integer] := Total[FactorInteger[n][[All, 2]]]
U[n_, m_] := (6 m + 1) (12 m + 1) Product[2^i 9 m + 1, {i, 1, n - 2}]
FindFirstChernickCarmichaelNumber[n_Integer?Positive] :=
Module[{step, i, m, formula, value},
step = Ceiling[2^(n - 4)];
If[n > 5, step *= 5];
i = step;
formula = U[n, m];
PrintTemporary[Dynamic[i]];
While[True,
value = formula /. m -> i;
If[PrimeFactorCounts[value] == n,
Break[];
];
i += step
];
{i, value}
]
FindFirstChernickCarmichaelNumber[3]
FindFirstChernickCarmichaelNumber[4]
FindFirstChernickCarmichaelNumber[5]
FindFirstChernickCarmichaelNumber[6]
FindFirstChernickCarmichaelNumber[7]
FindFirstChernickCarmichaelNumber[8]
FindFirstChernickCarmichaelNumber[9]
Output:
{1,1729}
{1,63973}
{380,26641259752490421121}
{380,1457836374916028334162241}
{780320,24541683183872873851606952966798288052977151461406721}
{950560,53487697914261966820654105730041031613370337776541835775672321}
{950560,58571442634534443082821160508299574798027946748324125518533225605795841}

Nim[edit]

Library: bignum

Until a(9) a simple primality test using divisions by odd numbers is sufficient. But for a(10), it is necessary to improve the test. We have used here some optimizations found in other solutions:

– eliminating multiples of 3, 5, 7, 11, 13, 17, 19, 23;
– using a probability test which implies to use big integers; so, we have to convert the tested number to a big integer;
– for n >= 5, checking only values of m which are multiple of 5 (in fact, we check only the multiples of 5 × 2^(n-4).

With these optimizations, the program executes in 4-5 minutes.

import strutils, sequtils
import bignum
 
const
Max = 10
Factors: array[3..Max, int] = [1, 1, 2, 4, 8, 16, 32, 64] # 1 for n=3 then 2^(n-4).
FirstPrimes = [3, 5, 7, 11, 13, 17, 19, 23]
 
#---------------------------------------------------------------------------------------------------
 
iterator factors(n, m: Natural): Natural =
## Yield the factors of U(n, m).
 
yield 6 * m + 1
yield 12 * m + 1
var k = 2
for _ in 1..(n - 2):
yield 9 * k * m + 1
inc k, k
 
#---------------------------------------------------------------------------------------------------
 
proc mayBePrime(n: int): bool =
## First primality test.
 
if n < 23: return true
 
for p in FirstPrimes:
if n mod p == 0:
return false
 
result = true
 
#---------------------------------------------------------------------------------------------------
 
proc isChernick(n, m: Natural): bool =
## Check if U(N, m) if a Chernick-Carmichael number.
 
# Use the first and quick test.
for factor in factors(n, m):
if not factor.mayBePrime():
return false
 
# Use the slow probability test (need to use a big int).
for factor in factors(n, m):
if probablyPrime(newInt(factor), 25) == 0:
return false
 
result = true
 
#---------------------------------------------------------------------------------------------------
 
proc a(n: Natural): tuple[m: Natural, factors: seq[Natural]] =
## For a given "n", find the smallest Charnick-Carmichael number.
 
var m: Natural = 0
var incr = (if n >= 5: 5 else: 1) * Factors[n] # For n >= 5, a(n) is a multiple of 5.
 
while true:
inc m, incr
if isChernick(n, m):
return (m, toSeq(factors(n, m)))
 
#———————————————————————————————————————————————————————————————————————————————————————————————————
 
import strformat
 
for n in 3..Max:
let (m, factors) = a(n)
 
stdout.write fmt"a({n}) = U({n}, {m}) = "
var s = ""
for factor in factors:
s.addSep(" × ")
s.add($factor)
stdout.write s, '\n'
Output:
a(3) = U(3, 1) = 7 × 13 × 19
a(4) = U(4, 1) = 7 × 13 × 19 × 37
a(5) = U(5, 380) = 2281 × 4561 × 6841 × 13681 × 27361
a(6) = U(6, 380) = 2281 × 4561 × 6841 × 13681 × 27361 × 54721
a(7) = U(7, 780320) = 4681921 × 9363841 × 14045761 × 28091521 × 56183041 × 112366081 × 224732161
a(8) = U(8, 950560) = 5703361 × 11406721 × 17110081 × 34220161 × 68440321 × 136880641 × 273761281 × 547522561
a(9) = U(9, 950560) = 5703361 × 11406721 × 17110081 × 34220161 × 68440321 × 136880641 × 273761281 × 547522561 × 1095045121
a(10) = U(10, 3208386195840) = 19250317175041 × 38500634350081 × 57750951525121 × 115501903050241 × 231003806100481 × 462007612200961 × 924015224401921 × 1848030448803841 × 3696060897607681 × 7392121795215361

PARI/GP[edit]

 
cherCar(n)={
my(C=vector(n));C[1]=6; C[2]=12; for(g=3,n,C[g]=2^(g-2)*9);
my(i=1); my(N(g)=while(i<=n&ispseudoprime(g*C[i]+1),i=i+1); return(i>n));
i=1; my(G(g)=while(i<=n&isprime(g*C[i]+1),i=i+1); return(i>n));
i=1; if(n>4,i=2^(n-4)); if(n>5,i=i*5); my(m=i); while(!(N(m)&G(m)),m=m+i);
printf("cherCar(%d): m = %d\n",n,m)}
for(x=3,9,cherCar(x))
 
Output:
cherCar(3): m = 1
cherCar(4): m = 1
cherCar(5): m = 380
cherCar(6): m = 380
cherCar(7): m = 780320
cherCar(8): m = 950560
cherCar(9): m = 950560
cherCar(10): m = 3208386195840

Perl[edit]

Library: ntheory
use 5.020;
use warnings;
use ntheory qw/:all/;
use experimental qw/signatures/;
 
sub chernick_carmichael_factors ($n, $m) {
(6*$m + 1, 12*$m + 1, (map { (1 << $_) * 9*$m + 1 } 1 .. $n-2));
}
 
sub chernick_carmichael_number ($n, $callback) {
 
my $multiplier = ($n > 4) ? (1 << ($n-4)) : 1;
 
for (my $m = 1 ; ; ++$m) {
my @f = chernick_carmichael_factors($n, $m * $multiplier);
next if not vecall { is_prime($_) } @f;
$callback->(@f);
last;
}
}
 
foreach my $n (3..9) {
chernick_carmichael_number($n, sub (@f) { say "a($n) = ", vecprod(@f) });
}
Output:
a(3) = 1729
a(4) = 63973
a(5) = 26641259752490421121
a(6) = 1457836374916028334162241
a(7) = 24541683183872873851606952966798288052977151461406721
a(8) = 53487697914261966820654105730041031613370337776541835775672321
a(9) = 58571442634534443082821160508299574798027946748324125518533225605795841

Phix[edit]

Library: Phix/mpfr
Translation of: Sidef
with javascript_semantics
function chernick_carmichael_factors(integer n, m)
    sequence res = {6*m + 1, 12*m + 1}
    for i=1 to n-2 do
        res &= power(2,i) * 9*m + 1
    end for
    return res
end function
 
include mpfr.e
mpz p = mpz_init()
 
function m_prime(atom a)
    mpz_set_d(p,a)
    return mpz_prime(p)
end function
 
function is_chernick_carmichael(integer n, m)
    return iff(n==2 ? m_prime(6*m + 1) and m_prime(12*m + 1)
                    : m_prime(power(2,n-2) * 9*m + 1) and 
                      is_chernick_carmichael(n-1, m))
end function
 
function chernick_carmichael_number(integer n)
    integer m = iff(n>4 ? power(2,n-4) : 1), mm = m
    while not is_chernick_carmichael(n, mm) do mm += m end while
    return {chernick_carmichael_factors(n, mm),mm}
end function
 
for n=3 to 9 do
    {sequence f, integer m} = chernick_carmichael_number(n)
    mpz_set_si(p,1)
    for i=1 to length(f) do
        mpz_mul_d(p,p,f[i])
        f[i] = sprintf("%d",f[i])
    end for
    printf(1,"U(%d,%d): %s = %s\n",{n,m,mpz_get_str(p),join(f," * ")})
end for
Output:
U(3,1): 1729 = 7 * 13 * 19
U(4,1): 63973 = 7 * 13 * 19 * 37
U(5,380): 26641259752490421121 = 2281 * 4561 * 6841 * 13681 * 27361
U(6,380): 1457836374916028334162241 = 2281 * 4561 * 6841 * 13681 * 27361 * 54721
U(7,780320): 24541683183872873851606952966798288052977151461406721 = 4681921 * 9363841 * 14045761 * 28091521 * 56183041 * 112366081 * 224732161
U(8,950560): 53487697914261966820654105730041031613370337776541835775672321 = 5703361 * 11406721 * 17110081 * 34220161 * 68440321 * 136880641 * 273761281 * 547522561
U(9,950560): 58571442634534443082821160508299574798027946748324125518533225605795841 = 5703361 * 11406721 * 17110081 * 34220161 * 68440321 * 136880641 * 273761281 * 547522561 * 1095045121

Pleasingly fast, note however that a(10) remains well out of reach / would probably need a complete rewrite.

with cheat[edit]

Translation of: C
with added cheat for the a(10) case - I found a nice big prime factor of k and added that on each iteration instead of 1.

You could also use the sequence {1,1,1,1,19,19,4877,457,457,12564169}, if you know a way to build that, and then it wouldn't be cheating anymore...

with javascript_semantics
include mpfr.e
sequence ppp = {3,5,7,11,13,17,19,23}
function primality_pretest(atom k)
    for i=1 to length(ppp) do
        if remainder(k,ppp[i])=0 then return (k<=23) end if
    end for
    return true
end function
 
function probprime(atom k, mpz n)
    mpz_set_d(n, k)
    return mpz_prime(n)
end function
 
function is_chernick(integer n, atom m, mpz z)
    atom t = 9 * m;
    if primality_pretest(6 * m + 1) == false then return false end if
    if primality_pretest(12 * m + 1) == false then return false end if
    for i=1 to n-3 do
        if primality_pretest(t*power(2,i) + 1) == false then return false end if
    end for
    if probprime(6 * m + 1, z) == false then return false end if
    if probprime(12 * m + 1, z) == false then return false end if
    for i=1 to n-2 do
        if probprime(t*power(2,i) + 1, z) == false then return false end if
    end for
    return true
end function
 
procedure main()
    atom t0 = time()
    mpz z = mpz_init(0)
 
    for n=3 to 10 do
        atom multiplier = iff(n>4 ? power(2,n-4) : 1), k = 1
        if n>5 then multiplier *= 5 end if
 
        while true do
            if n=10 then k += 12564168 end if   -- cheat!
            atom m = k * multiplier;
            if is_chernick(n, m, z) then
                printf(1,"a(%d) has m = %d\n", {n, m})
                exit
            end if
            k += 1
        end while
    end for
    ?elapsed(time()-t0)
end procedure
main()
Output:
a(3) has m = 1
a(4) has m = 1
a(5) has m = 380
a(6) has m = 380
a(7) has m = 780320
a(8) has m = 950560
a(9) has m = 950560
a(10) has m = 3208386195840
"0.1s"

Prolog[edit]

SWI Prolog is too slow to solve for a(10), even with optimizing by increasing the multiplier and implementing a trial division check. (actually, my implementation of Miller-Rabin in Prolog already starts with a trial division by small primes.)

 
?- use_module(library(primality)).
 
u(3, M, A * B * C) :-
A is 6*M + 1, B is 12*M + 1, C is 18*M + 1, !.
u(N, M, U0 * D) :-
succ(Pn, N), u(Pn, M, U0),
D is 9*(1 << (N - 2))*M + 1.
 
prime_factorization(A*B) :- prime(B), prime_factorization(A), !.
prime_factorization(A) :- prime(A).
 
step(N, 1) :- N < 5, !.
step(5, 2) :- !.
step(N, K) :- K is 5*(1 << (N - 4)).
 
a(N, Factors) :- % due to backtracking nature of Prolog, a(n) will return all chernick-carmichael numbers.
N > 2, !,
step(N, I),
between(1, infinite, J), M is I * J,
u(N, M, Factors),
prime_factorization(Factors).
 
main :-
forall(
(between(3, 9, K), once(a(K, Factorization)), N is Factorization),
format("~w: ~w = ~w~n", [K, Factorization, N])),
halt.
 
?- main.
 

isprime predicate:

 
prime(N) :-
integer(N),
N > 1,
divcheck(
N,
[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31,
37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79,
83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137,
139, 149],
Result),
((Result = prime, !); miller_rabin_primality_test(N)).
 
divcheck(_, [], unknown) :- !.
divcheck(N, [P|_], prime) :- P*P > N, !.
divcheck(N, [P|Ps], State) :- N mod P =\= 0, divcheck(N, Ps, State).
 
miller_rabin_primality_test(N) :-
bases(Bases, N),
forall(member(A, Bases), strong_fermat_pseudoprime(N, A)).
 
miller_rabin_precision(16).
 
bases([31, 73], N) :- N < 9_080_191, !.
bases([2, 7, 61], N) :- N < 4_759_123_141, !.
bases([2, 325, 9_375, 28_178, 450_775, 9_780_504, 1_795_265_022], N) :-
N < 18_446_744_073_709_551_616, !. % 2^64
bases(Bases, N) :-
miller_rabin_precision(T), RndLimit is N - 2,
length(Bases, T), maplist(random_between(2, RndLimit), Bases).
 
strong_fermat_pseudoprime(N, A) :- % miller-rabin strong pseudoprime test with base A.
succ(Pn, N), factor_2s(Pn, S, D),
X is powm(A, D, N),
((X =:= 1, !); \+ composite_witness(N, S, X)).
 
composite_witness(_, 0, _) :- !.
composite_witness(N, K, X) :-
X =\= N-1,
succ(Pk, K), X2 is (X*X) mod N, composite_witness(N, Pk, X2).
 
factor_2s(N, S, D) :- factor_2s(0, N, S, D).
factor_2s(S, D, S, D) :- D /\ 1 =\= 0, !.
factor_2s(S0, D0, S, D) :-
succ(S0, S1), D1 is D0 >> 1,
factor_2s(S1, D1, S, D).
 
Output:
3: 7*13*19 = 1729
4: 7*13*19*37 = 63973
5: 2281*4561*6841*13681*27361 = 26641259752490421121
6: 2281*4561*6841*13681*27361*54721 = 1457836374916028334162241
7: 4681921*9363841*14045761*28091521*56183041*112366081*224732161 = 24541683183872873851606952966798288052977151461406721
8: 5703361*11406721*17110081*34220161*68440321*136880641*273761281*547522561 = 53487697914261966820654105730041031613370337776541835775672321
9: 5703361*11406721*17110081*34220161*68440321*136880641*273761281*547522561*1095045121 = 58571442634534443082821160508299574798027946748324125518533225605795841

Python[edit]

 
"""
 
Python implementation of
http://rosettacode.org/wiki/Chernick%27s_Carmichael_numbers
 
"""

 
# use sympy for prime test
 
from sympy import isprime
 
# based on C version
 
def primality_pretest(k):
if not (k % 3) or not (k % 5) or not (k % 7) or not (k % 11) or not(k % 13) or not (k % 17) or not (k % 19) or not (k % 23):
return (k <= 23)
 
return True
 
def is_chernick(n, m):
 
t = 9 * m
 
if not primality_pretest(6 * m + 1):
return False
 
if not primality_pretest(12 * m + 1):
return False
 
for i in range(1,n-1):
if not primality_pretest((t << i) + 1):
return False
 
if not isprime(6 * m + 1):
return False
 
if not isprime(12 * m + 1):
return False
 
for i in range(1,n - 1):
if not isprime((t << i) + 1):
return False
 
return True
 
for n in range(3,10):
 
if n > 4:
multiplier = 1 << (n - 4)
else:
multiplier = 1
 
if n > 5:
multiplier *= 5
 
 
k = 1
 
while True:
m = k * multiplier
 
if is_chernick(n, m):
print("a("+str(n)+") has m = "+str(m))
break
 
k += 1
 
Output:
a(3) has m = 1
a(4) has m = 1
a(5) has m = 380
a(6) has m = 380
a(7) has m = 780320
a(8) has m = 950560
a(9) has m = 950560

Raku[edit]

(formerly Perl 6)

Works with: Rakudo version 2019.03
Translation of: Perl

Use the ntheory library from Perl 5 for primality testing since it is much, much faster than Rakus built-in .is-prime method.

use Inline::Perl5;
use ntheory:from<Perl5> <:all>;
 
sub chernick-factors ($n, $m) {
6*$m + 1, 12*$m + 1, |((1 .. $n-2).map: { (1 +< $_) * 9*$m + 1 } )
}
 
sub chernick-carmichael-number ($n) {
 
my $multiplier = 1 +< (($n-4) max 0);
my $iterator = $n < 5 ?? (1 .. *) !! (1 .. *).map: * * 5;
 
$multiplier * $iterator.first: -> $m {
[&&] chernick-factors($n, $m * $multiplier).map: { is_prime($_) }
}
 
}
 
for 3 .. 9 -> $n {
my $m = chernick-carmichael-number($n);
my @f = chernick-factors($n, $m);
say "U($n, $m): {[*] @f} = {@f.join(' ⨉ ')}";
}
Output:
U(3, 1): 1729 = 7 ⨉ 13 ⨉ 19
U(4, 1): 63973 = 7 ⨉ 13 ⨉ 19 ⨉ 37
U(5, 380): 26641259752490421121 = 2281 ⨉ 4561 ⨉ 6841 ⨉ 13681 ⨉ 27361
U(6, 380): 1457836374916028334162241 = 2281 ⨉ 4561 ⨉ 6841 ⨉ 13681 ⨉ 27361 ⨉ 54721
U(7, 780320): 24541683183872873851606952966798288052977151461406721 = 4681921 ⨉ 9363841 ⨉ 14045761 ⨉ 28091521 ⨉ 56183041 ⨉ 112366081 ⨉ 224732161
U(8, 950560): 53487697914261966820654105730041031613370337776541835775672321 = 5703361 ⨉ 11406721 ⨉ 17110081 ⨉ 34220161 ⨉ 68440321 ⨉ 136880641 ⨉ 273761281 ⨉ 547522561
U(9, 950560): 58571442634534443082821160508299574798027946748324125518533225605795841 = 5703361 ⨉ 11406721 ⨉ 17110081 ⨉ 34220161 ⨉ 68440321 ⨉ 136880641 ⨉ 273761281 ⨉ 547522561 ⨉ 1095045121

Sidef[edit]

func chernick_carmichael_factors (n, m) {
[6*m + 1, 12*m + 1, {|i| 2**i * 9*m + 1 }.map(1 .. n-2)...]
}
 
func is_chernick_carmichael (n, m) {
(n == 2) ? (is_prime(6*m + 1) && is_prime(12*m + 1))
 : (is_prime(2**(n-2) * 9*m + 1) && __FUNC__(n-1, m))
}
 
func chernick_carmichael_number(n, callback) {
var multiplier = (n>4 ? 2**(n-4) : 1)
var m = (1..Inf -> first {|m| is_chernick_carmichael(n, m * multiplier) })
var f = chernick_carmichael_factors(n, m * multiplier)
callback(f...)
}
 
for n in (3..9) {
chernick_carmichael_number(n, {|*f| say "a(#{n}) = #{f.join(' * ')}" })
}
Output:
a(3) = 7 * 13 * 19
a(4) = 7 * 13 * 19 * 37
a(5) = 2281 * 4561 * 6841 * 13681 * 27361
a(6) = 2281 * 4561 * 6841 * 13681 * 27361 * 54721
a(7) = 4681921 * 9363841 * 14045761 * 28091521 * 56183041 * 112366081 * 224732161
a(8) = 5703361 * 11406721 * 17110081 * 34220161 * 68440321 * 136880641 * 273761281 * 547522561
a(9) = 5703361 * 11406721 * 17110081 * 34220161 * 68440321 * 136880641 * 273761281 * 547522561 * 1095045121

Wren[edit]

Translation of: Go
Library: Wren-big
Library: Wren-fmt

Based on Go's 'more efficient' version. Reaches a(9) in just over 0.1 seconds but a(10) would still be out of reasonable reach for Wren so I've had to be content with that.

import "/big" for BigInt, BigInts
import "/fmt" for Fmt
 
var min = 3
var max = 9
var prod = BigInt.zero
var fact = BigInt.zero
var factors = List.filled(max, 0)
var bigFactors = List.filled(max, null)
 
var init = Fn.new {
for (i in 0...max) bigFactors[i] = BigInt.zero
}
 
var isPrimePretest = Fn.new { |k|
if (k%3 == 0 || k%5 == 0 || k%7 == 0 || k%11 == 0 ||
(k%13 == 0) || k%17 == 0 || k%19 == 0 || k%23 == 0) return k <= 23
return true
}
 
var ccFactors = Fn.new { |n, m|
if (!isPrimePretest.call(6*m + 1)) return false
if (!isPrimePretest.call(12*m + 1)) return false
factors[0] = 6*m + 1
factors[1] = 12*m + 1
var t = 9 * m
var i = 1
while (i <= n-2) {
var tt = (t << i) + 1
if (!isPrimePretest.call(tt)) return false
factors[i+1] = tt
i = i + 1
}
for (i in 0...n) {
fact = BigInt.new(factors[i])
if (!fact.isProbablePrime(1)) return false
bigFactors[i] = fact
}
return true
}
 
var ccNumbers = Fn.new { |start, end|
for (n in start..end) {
var mult = 1
if (n > 4) mult = 1 << (n - 4)
if (n > 5) mult = mult * 5
var m = mult
while (true) {
if (ccFactors.call(n, m)) {
var num = BigInts.prod(bigFactors.take(n))
Fmt.print("a($d) = $i", n, num)
Fmt.print("m($d) = $d", n, m)
Fmt.print("Factors: $n\n", factors[0...n])
break
}
m = m + mult
}
}
}
 
init.call()
ccNumbers.call(min, max)
Output:
a(3) = 1729
m(3) = 1
Factors: [7, 13, 19]

a(4) = 63973
m(4) = 1
Factors: [7, 13, 19, 37]

a(5) = 26641259752490421121
m(5) = 380
Factors: [2281, 4561, 6841, 13681, 27361]

a(6) = 1457836374916028334162241
m(6) = 380
Factors: [2281, 4561, 6841, 13681, 27361, 54721]

a(7) = 24541683183872873851606952966798288052977151461406721
m(7) = 780320
Factors: [4681921, 9363841, 14045761, 28091521, 56183041, 112366081, 224732161]

a(8) = 53487697914261966820654105730041031613370337776541835775672321
m(8) = 950560
Factors: [5703361, 11406721, 17110081, 34220161, 68440321, 136880641, 273761281, 547522561]

a(9) = 58571442634534443082821160508299574798027946748324125518533225605795841
m(9) = 950560
Factors: [5703361, 11406721, 17110081, 34220161, 68440321, 136880641, 273761281, 547522561, 1095045121]

zkl[edit]

Translation of: Go
Library: GMP
GNU Multiple Precision Arithmetic Library

Using GMP (probabilistic primes), because it is easy and fast to check primeness.

var [const] BI=Import("zklBigNum");  // libGMP
 
fcn ccFactors(n,m){ // not re-entrant
prod:=BI(6*m + 1);
if(not prod.probablyPrime()) return(False);
fact:=BI(12*m + 1);
if(not fact.probablyPrime()) return(False);
prod.mul(fact);
foreach i in ([1..n-2]){
fact.set((2).pow(i) *9*m + 1);
if(not fact.probablyPrime()) return(False);
prod.mul(fact);
}
prod
}
 
fcn ccNumbers(start,end){
foreach n in ([start..end]){
a,m := ( if(n<=4) 1 else (2).pow(n - 4) ), a;
while(1){
if(num := ccFactors(n,m)){
println("a(%d) = %,d".fmt(n,num));
break;
}
m+=a;
}
}
}
ccNumbers(3,9);
Output:
a(3) = 1,729
a(4) = 63,973
a(5) = 26,641,259,752,490,421,121
a(6) = 1,457,836,374,916,028,334,162,241
a(7) = 24,541,683,183,872,873,851,606,952,966,798,288,052,977,151,461,406,721
a(8) = 53,487,697,914,261,966,820,654,105,730,041,031,613,370,337,776,541,835,775,672,321
a(9) = 58,571,442,634,534,443,082,821,160,508,299,574,798,027,946,748,324,125,518,533,225,605,795,841