Primality of numbers of the form 2n^2-1 (Euler, 216)

/* ### Project Euler, problem 216 ### */
/* Notice that I'm not the best C programmer ever :D */

#include <stdio.h>
#include <tgmath.h>

#define MAX 50000000

short isPrime(unsigned long n) {
    int i;
    for(i=2; i<=(unsigned long) trunc(sqrt((double) n)); ++i)
        if (n % i == 0) return 0;
    return 1;
}

unsigned long t(unsigned long n){
    return 2 * n*n - 1;
}

int main(void) {
    unsigned long i, count=0;
    for(i=2; i<=MAX; ++i)
        if (isPrime(t(i)))
            ++count;
    printf("%lu", count);
    return 0;
}