Expansion of g.f.: x/((1-x^2)^4 -1+x).

Terms

a(0) =1a(1) =4a(2) =16a(3) =58a(4) =208a(5) =740a(6) =2628a(7) =9327a(8) =33096a(9) =117432a(10) =416668a(11) =1478400a(12) =5245576a(13) =18612052a(14) =66038209a(15) =234312956a(16) =831375680a(17) =2949839102a(18) =10466448480a(19) =37136447100a(20) =131765393560a(21) =467522347871