268435498domain: NAppears in sequencesNumber of connected induced (non-null) subgraphs of the friendship graph with 2n+1 nodes.at n=13A286186a(0) = 4; to obtain a(k), write out the base-(2^k) expansion of a(k-1), bump to base 2^(k+1), then subtract 1.at n=26A372237