Roger Hui wrote: > The MathWorld page doesn't give the derivation
I believe the MathWorld formula is the same as the J derivation. Primes are indexed starting at 1. You iterate over primes p_k whose square is less than or equal to n. The term pi(n/p_k) counts primes q such that n>:(p_k)*q . You subtract (k-1) to count only primes q with p_k<q . Best wishes, John ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
