a(n) = T(2n,n), T given by A026747.

Terms

a(0) =1a(1) =3a(2) =11a(3) =44a(4) =184a(5) =790a(6) =3452a(7) =15278a(8) =68290a(9) =307696a(10) =1395696a(11) =6367199a(12) =29193025a(13) =134442102a(14) =621609060a(15) =2884432810a(16) =13428450520a(17) =62703991531a(18) =293606387095