finding biggest prime factor

Check out the following question:

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Prime factors of 40 are 2,5.

Find the biggest prime factor of 600851475143. 

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I wrote the following code in C but it does not work. Please share your ideas.

/* Program To calculate largest prime factor of  600851475143
*    working simultaneously with long and double types is hard. this number
*     doesn't fit in a long type variable. so we have to take another measure.
*     maybe creating prime numbers and storing them in an array will be better.
*/

#include<stdio.h>
#include<math.h>


int IsPrime(unsigned long num)
{
    int index;
    int count=0; /* num itself and number One is not counted, so count==0 after loop determines answer*/
    /* index from num/2 because a numbers factors are not bigger than its half*/
    for(index=sqrt(num);index>1;index--)
        if(num%index==0) count++;
    return count==0?1:0;
}
   
   

int main()
{
    const  double LargeNum=600851475143;
    int largestPrimeFactor;
    unsigned long i;
    printf("LargeNum: %f",LargeNum);
    /* double sqrtLargeNum=sqrt(LargeNum);
    printf("sqrt: %f\n",sqrtLargeNum);
    */
    printf("long i: %u",i);
    for(i=LargeNum/2;i>1;i--)
//        if(LargeNum%i==0) /* Is i a factor? */
            if(IsPrime(i)){     /* Is i a prime? */
                largestPrimeFactor=i;
                break;
            }
           
       
    printf("Largest Prime Factor: %d\n",largestPrimeFactor);
   
   
    return 0;
}