G.f.: Sum_{n>=0} x^n / (1-x^2)^(2*n+1) * [Sum_{k=0..n} C(n,k)^2 * x^(2*k)]^2.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =15a(5) =39a(6) =116a(7) =340a(8) =1001a(9) =3041a(10) =9322a(11) =28718a(12) =89363a(13) =279987a(14) =881376a(15) =2788464a(16) =8860677a(17) =28256709a(18) =90407666a(19) =290124182a(20) =933482527a(21) =3010689527a(22) =9731366060a(23) =31516942060a(24) =102259648701a(25) =332347297141