Data commit

Task/Ultra-useful-primes/C/ultra-useful-primes.c

@@ -0,0 +1,22 @@
+#include <stdio.h>
+#include <gmp.h>
+
+int a(unsigned int n) {
+    int k;
+    mpz_t p;
+    mpz_init_set_ui(p, 1);
+    mpz_mul_2exp(p, p, 1 << n);
+    mpz_sub_ui(p, p, 1);
+    for (k = 1; ; k += 2) {
+        if (mpz_probab_prime_p(p, 15) > 0) return k;
+        mpz_sub_ui(p, p, 2);
+    }
+}
+
+int main() {
+    unsigned int n;
+    printf(" n   k\n");
+    printf("----------\n");
+    for (n = 1; n < 15; ++n) printf("%2d   %d\n", n, a(n));
+    return 0;
+}