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381.c
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381.c
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#include <stdlib.h>
#include <stdbool.h>
#include <math.h>
#include <stdio.h>
int fast_exp_mod(int base, int power, int divisor) {
if (power == 0) {
return 1;
} else if (power % 2 == 0) {
return fast_exp_mod((long) base * base % divisor, power / 2, divisor) % divisor;
} else {
return (long) base * fast_exp_mod(base, power - 1, divisor) % divisor;
}
}
int mod_inverse(int n, int divisor) {
return fast_exp_mod(n, divisor - 2, divisor);
}
int specific_factorial_sum_mod(int p) {
return ((mod_inverse(8, p) * -3) + p * 3) % p;
}
int main(void) {
int MAX = 1e8;
bool* prime = malloc(MAX * sizeof(bool));
for (int i = 0; i < MAX; i++) {
prime[i] = i > 1;
}
for (int i = 2; i < sqrt(MAX); i++) {
if (prime[i]) {
for (int n = i * i; n < MAX; n += i) {
prime[n] = false;
}
}
}
long sum = 0;
for (int i = 5; i < MAX; i++) {
if (prime[i]) {
sum += specific_factorial_sum_mod(i);
}
}
// doesn't work?!
printf ("Sum: %ld\n", sum);
return 0;
}