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

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =4a(5) =9a(6) =19a(7) =47a(8) =113a(9) =287a(10) =733a(11) =1920a(12) =5064a(13) =13557a(14) =36553a(15) =99455a(16) =272293a(17) =750262a(18) =2077751a(19) =5781971a(20) =16156866a(21) =45321635a(22) =127566689a(23) =360191846a(24) =1019926954a(25) =2895648896a(26) =8240832570a(27) =23505344031a(28) =67183161970a(29) =192393195097