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Can two neighbouring primes average to a prime

Question

Take two primes p<qp<q with no prime strictly between them. Is it possible for p+q2\tfrac{p+q}{2} to be prime?

Short answer

No. For p=2,q=3p=2,q=3 the mean 2.52.5 is not an integer. Otherwise the mean is an integer lying strictly between pp and qq, and since p,qp,q are consecutive primes, no integer in that gap is prime.

Adapted from Crack, Heard on the Street, Q1.68, p.21; question and answer rewritten by SelfMadeQuant.com.

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