a(n) = Sum_{k=0..n} T(n,k) * T(n,2n-k), with T given by A027023.

Terms

a(0) =1a(1) =2a(2) =5a(3) =20a(4) =77a(5) =306a(6) =1209a(7) =4656a(8) =17713a(9) =66618a(10) =248025a(11) =916020a(12) =3359789a(13) =12250026a(14) =44435997a(15) =160466304a(16) =577185745a(17) =2068826290a(18) =7392167585a(19) =26338879556a(20) =93609302941a(21) =331924381218