T(2n,n), T given by A026692.

Terms

a(0) =1a(1) =2a(2) =8a(3) =30a(4) =138a(5) =562a(6) =2700a(7) =11364a(8) =55756a(9) =238874a(10) =1186616a(11) =5140518a(12) =25745026a(13) =112379472a(14) =566088616a(15) =2484706008a(16) =12570242776a(17) =55405777178a(18) =281236520744