sibsutis/proglabs/lab6/9.c

#include <stdio.h>
int pr(long long n);

int main() {
    long long int x,i;
    int y = 1;
    if(!(scanf("%lld",&x)))return 1;

    for(i = x; i > 1; --i)
        if(pr(i))
            printf("%lld\t",i);

    return 0;
}

int pr(long long x) {
    if (x < 2) return 0;
    if (x == 2 || x == 3) return 1;
    if (x % 2 == 0 || x % 3 == 0) return 0;

    for (long long i = 5; i * i <= x; i += 6) {
        if (x % i == 0 || x % (i + 2) == 0)
            return 0;
    }
    return 1;
}