a(n)=T(2n,n+1), where T is given by A048113.

Terms

a(0) =0a(1) =0a(2) =2a(3) =6a(4) =24a(5) =84a(6) =312a(7) =1140a(8) =4248a(9) =15852a(10) =59688a(11) =225624a(12) =857424a(13) =3270216a(14) =12518472a(15) =48062520a(16) =185034360a(17) =714038448a(18) =2761343064a(19) =10698992304a(20) =41525207448a(21) =161418403008a(22) =628356425856