G.f. A(x) satisfies A(x) = 1 + x/(1-x^2)^2 * A(x)^2.

Terms

a(0) =1a(1) =1a(2) =2a(3) =7a(4) =22a(5) =75a(6) =264a(7) =958a(8) =3558a(9) =13463a(10) =51722a(11) =201223a(12) =791192a(13) =3139131a(14) =12552204a(15) =50533205a(16) =204654950a(17) =833219825a(18) =3408319290a(19) =14000834736a(20) =57732586382a(21) =238884001905a(22) =991558312956