Massive Prime Number!

Computer Science Level pending

Let $a$ be the $100^\text{th}$ largest prime number that is less than $10^7$ . What are the last 4 digits of $a$ ?

The answer is 8279.

1 solution

First Last
Mar 2, 2017

bool prime ;
int count = 0 ;
for(int i = pow(10,7) - 1 ; ; i -= 2){ //list numbers possibly prime
   prime = true ;
     for(int a = 3 ; a < sqrt(i) ; a += 2){ //check relevant factors
         if(i%a == 0){
             prime = false ;
             break ;
         }
     }
     if(prime) count ++ ;
     if(count == 100){
         cout << i << endl ;
         break ;
     }
 }