a(n) = Sum{T(n,k)*T(n,2n-k)}, 0<=k<=n-1, T given by A027926.

Terms

a(0) =1a(1) =3a(2) =12a(3) =40a(4) =130a(5) =404a(6) =1227a(7) =3653a(8) =10720a(9) =31090a(10) =89316a(11) =254568a(12) =720757a(13) =2029095a(14) =5684340a(15) =15855964a(16) =44061862a(17) =122032508a(18) =336966015a(19) =927953705a(20) =2549229256a(21) =6987648358a(22) =19115124552a(23) =52194037200a(24) =142274514025a(25) =387215773899