G.f. satisfies A(x) = 1 + x*cycle_index(Cyclic(4), A(x)).

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =7a(5) =22a(6) =65a(7) =212a(8) =697a(9) =2372a(10) =8179a(11) =28703a(12) =101851a(13) =365393a(14) =1322034a(15) =4820378a(16) =17691018a(17) =65306700a(18) =242317423a(19) =903242615a(20) =3380707920a(21) =12700575742a(22) =47874113728a(23) =181013422393a(24) =686341240781