630375241380domain: NAppears in sequencesNumber of restricted hexagonal polyominoes with n cells.at n=20A002212a(n) = a(1)*a(n-1) + a(2)*a(n-2) + ...+ a(n-2)*a(2) for n >= 3.at n=20A025238G.f. satisfies A(x) = (1 + x) * (1 + x/A(x)).at n=21A366325