a(n) = A026615(2*n, n).

Terms

a(0) =1a(1) =3a(2) =10a(3) =34a(4) =120a(5) =434a(6) =1596a(7) =5940a(8) =22308a(9) =84370a(10) =320892a(11) =1226108a(12) =4702880a(13) =18097044a(14) =69832600a(15) =270118440a(16) =1047043260a(17) =4066132050a(18) =15816664380a(19) =61615392300a(20) =240347793840a(21) =938669220060