a(n) = A026626(2*n, n-1).

Terms

a(0) =1a(1) =6a(2) =21a(3) =79a(4) =296a(5) =1117a(6) =4237a(7) =16147a(8) =61782a(9) =237208a(10) =913466a(11) =3526826a(12) =13647886a(13) =52920075a(14) =205566205a(15) =799791235a(16) =3116196550a(17) =12157265980a(18) =47485135510a(19) =185671296850a(20) =726703966600