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

Terms

a(0) =1a(1) =1a(2) =8a(3) =22a(4) =121a(5) =406a(6) =2155a(7) =7624a(8) =40717a(9) =147001a(10) =792351a(11) =2892044a(12) =15703156a(13) =57728737a(14) =315180458a(15) =1164727748a(16) =6385672193a(17) =23691834033a(18) =130316812494a(19) =485018155062