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

Terms

a(0) =1a(1) =2a(2) =9a(3) =24a(4) =88a(5) =280a(6) =1064a(7) =3672a(8) =14456a(9) =52184a(10) =210504a(11) =782232a(12) =3210904a(13) =12176792a(14) =50638440a(15) =194956248a(16) =818961080a(17) =3189915224a(18) =13508052104a(19) =53105011480a(20) =226355549400a(21) =896636646936