12270214441domain: NAppears in sequencesNumber of perfect matchings (or domino tilings) in C_4 X P_n.at n=18A006253Indices of octagonal numbers which are also squares.at n=9A046184G.f.: -(2+7*x-x^3)/(1+4*x-4*x^3-x^4).at n=19A097949