Wagstaff primes: Difference between revisions

=={{header|C++}}==

{{libheader|GMP}}

{{libheader|Primesieve}}

<syntaxhighlight lang="cpp">#include <gmpxx.h>

#include <primesieve.hpp>

#include <iostream>

using big_int = mpz_class;

std::string to_string(const big_int& num, size_t n) {

std::string str = num.get_str();

size_t len = str.size();

if (len > n) {

str = str.substr(0, n / 2) + "..." + str.substr(len - n / 2);

str += " (";

str += std::to_string(len);

str += " digits)";

}

return str;

}

bool is_probably_prime(const big_int& n) {

return mpz_probab_prime_p(n.get_mpz_t(), 25) != 0;

}

int main() {

const big_int one(1);

primesieve::iterator pi;

pi.next_prime();

for (int i = 0; i < 24;) {

uint64_t p = pi.next_prime();

big_int n = ((one << p) + 1) / 3;

if (is_probably_prime(n))

std::cout << ++i << ": " << p << " - " << to_string(n, 30) << '\n';

}

}</syntaxhighlight>

{{out}}

<pre>

1: 3 - 3

2: 5 - 11

3: 7 - 43

4: 11 - 683

5: 13 - 2731

6: 17 - 43691

7: 19 - 174763

8: 23 - 2796203

9: 31 - 715827883

10: 43 - 2932031007403

11: 61 - 768614336404564651

12: 79 - 201487636602438195784363

13: 101 - 845100400152152934331135470251

14: 127 - 567137278201564...101238628035243 (38 digits)

15: 167 - 623574031927851...838653121833643 (50 digits)

16: 191 - 104618362256444...574077339085483 (58 digits)

17: 199 - 267823007376498...963798805883563 (60 digits)

18: 313 - 556246623937737...099018130434731 (94 digits)

19: 347 - 955624423329196...712921903606443 (104 digits)

20: 701 - 350675726769891...862167823854251 (211 digits)

21: 1709 - 961925272487010...857299070528171 (514 digits)

22: 2617 - 208150470990263...847933435947691 (788 digits)

23: 3539 - 737960982013072...304686486497963 (1065 digits)

24: 5807 - 401849623734300...466686663568043 (1748 digits)

</pre>