a(n) = T(2*n, n-2), where T is given by A026519.

Terms

a(0) =1a(1) =3a(2) =14a(3) =55a(4) =231a(5) =952a(6) =3976a(7) =16614a(8) =69750a(9) =293557a(10) =1238952a(11) =5240599a(12) =22212645a(13) =94318875a(14) =401143304a(15) =1708558480a(16) =7286677479a(17) =31113264579a(18) =132994055090a(19) =569048532612