# Algebra

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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 -

since n^2-1 = (n-1)(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 n-1 and n+1)