Factorial primes: Difference between revisions
Added C++ solution
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9: 12! - 1 = 479001599
10: 14! - 1 = 87178291199
</pre>
=={{header|C++}}==
<syntaxhighlight lang="cpp">#include <iomanip>
#include <iostream>
#include <gmpxx.h>
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() {
big_int f = 1;
for (int i = 0, n = 1; i < 31; ++n) {
f *= n;
if (is_probably_prime(f - 1)) {
++i;
std::cout << std::setw(2) << i << ": " << std::setw(3) << n
<< "! - 1 = " << to_string(f - 1, 40) << '\n';
}
if (is_probably_prime(f + 1)) {
++i;
std::cout << std::setw(2) << i << ": " << std::setw(3) << n
<< "! + 1 = " << to_string(f + 1, 40) << '\n';
}
}
}</syntaxhighlight>
{{out}}
<pre>
1: 1! + 1 = 2
2: 2! + 1 = 3
3: 3! - 1 = 5
4: 3! + 1 = 7
5: 4! - 1 = 23
6: 6! - 1 = 719
7: 7! - 1 = 5039
8: 11! + 1 = 39916801
9: 12! - 1 = 479001599
10: 14! - 1 = 87178291199
11: 27! + 1 = 10888869450418352160768000001
12: 30! - 1 = 265252859812191058636308479999999
13: 32! - 1 = 263130836933693530167218012159999999
14: 33! - 1 = 8683317618811886495518194401279999999
15: 37! + 1 = 13763753091226345046...79581580902400000001 (44 digits)
16: 38! - 1 = 52302261746660111176...24100074291199999999 (45 digits)
17: 41! + 1 = 33452526613163807108...40751665152000000001 (50 digits)
18: 73! + 1 = 44701154615126843408...03680000000000000001 (106 digits)
19: 77! + 1 = 14518309202828586963...48000000000000000001 (114 digits)
20: 94! - 1 = 10873661566567430802...99999999999999999999 (147 digits)
21: 116! + 1 = 33931086844518982011...00000000000000000001 (191 digits)
22: 154! + 1 = 30897696138473508879...00000000000000000001 (272 digits)
23: 166! - 1 = 90036917057784373664...99999999999999999999 (298 digits)
24: 320! + 1 = 21161033472192524829...00000000000000000001 (665 digits)
25: 324! - 1 = 22889974601791023211...99999999999999999999 (675 digits)
26: 340! + 1 = 51008644721037110809...00000000000000000001 (715 digits)
27: 379! - 1 = 24840307460964707050...99999999999999999999 (815 digits)
28: 399! + 1 = 16008630711655973815...00000000000000000001 (867 digits)
29: 427! + 1 = 29063471769607348411...00000000000000000001 (940 digits)
30: 469! - 1 = 67718096668149510900...99999999999999999999 (1051 digits)
31: 546! - 1 = 14130200926141832545...99999999999999999999 (1260 digits)
</pre>
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