a(n) = T(2n,n), where T is the array defined in A026082.

Terms

a(0) =1a(1) =2a(2) =4a(3) =16a(4) =93a(5) =518a(6) =2932a(7) =16709a(8) =95849a(9) =552749a(10) =3201848a(11) =18616751a(12) =108592239a(13) =635175153a(14) =3724193120a(15) =21881993115a(16) =128810517855a(17) =759515272026