Expansion of g.f.: x/((1-x^2)^3 -1+x).

Terms

a(0) =1a(1) =3a(2) =9a(3) =24a(4) =63a(5) =163a(6) =420a(7) =1080a(8) =2775a(9) =7128a(10) =18307a(11) =47016a(12) =120744a(13) =310086a(14) =796338a(15) =2045089a(16) =5252025a(17) =13487805a(18) =34638234a(19) =88954965a(20) =228446569a(21) =586677030a(22) =1506654000a(23) =3869260527a(24) =9936705456a(25) =25518600937a(26) =65534698260a(27) =168300632412a(28) =432215354952