a(1)=2; to form a(n+1), append P, the largest prime < a(n), if any and Q, the smallest prime > a(n), at the beginning and the end of a(n), respectively.

A067837

a(1)=2; to form a(n+1), append P, the largest prime < a(n), if any and Q, the smallest prime > a(n), at the beginning and the end of a(n), respectively.