Check if a number can be written as a sum of ' k ' prime numbers | Checking if a number is prime or not ; check for numbers from 2 to sqrt ( x ) if it is divisible return false ; Returns true if N can be written as sum of K primes ; N < 2 K directly return false ; If K = 1 return value depends on primality of N ; if N is even directly return true ; ; If N is odd , then one prime must be 2. All other primes are odd and cannot have a pair sum as even . ; If K >= 3 return true ; ; Driver Code
< ? php function isprime ( $ x ) { for ( $ i = 2 ; $ i * $ i <= $ x ; $ i ++ ) if ( $ x % $ i == 0 ) return false ; return true ; } function isSumOfKprimes ( $ N , $ K ) { if ( $ N < 2 * $ K ) return false ; if ( $ K == 1 ) return isprime ( $ N ) ; if ( $ K == 2 ) { if ( $ N % 2 == 0 ) return true ; return isprime ( $ N - 2 ) ; } return true ; } $ n = 10 ; $ k = 2 ; if ( isSumOfKprimes ( $ n , $ k ) ) echo " Yes " ; else echo " No " ; ? >