a(n) = T(2*n-1, n-2), where T is given by A026519.

Terms

a(0) =1a(1) =3a(2) =13a(3) =50a(4) =205a(5) =833a(6) =3437a(7) =14232a(8) =59301a(9) =248050a(10) =1041469a(11) =4385888a(12) =18519306a(13) =78376403a(14) =332370925a(15) =1412000824a(16) =6008104249a(17) =25601113893a(18) =109229104313a(19) =466577280830