a(n) = T(2*n-1, n-2), where T is given by A026584.

Terms

a(0) =1a(1) =2a(2) =13a(3) =42a(4) =225a(5) =802a(6) =4235a(7) =15478a(8) =82425a(9) =304156a(10) =1634435a(11) =6064389a(12) =32819839a(13) =122244344a(14) =665162897a(15) =2484851486a(16) =13577768505a(17) =50841782786a(18) =278745377821