Posted by Jack on Tuesday, March 26, 2013 at 10:06am.
What is the sum of all integer values of n satisfying 1 \leq n \leq 100, such that n^2  1 is a product of exactly two distinct prime numbers?

Algebra  Steve, Tuesday, March 26, 2013 at 10:41am
since n^21 = (n1)(n+1) we are looking for twin primes
Just look up a list of twin primes, and sum their average (since you want the even number n between n1 and n+1)