a(n) = T(2n,n-1), T given by A026780.

Terms

a(0) =1a(1) =7a(2) =40a(3) =217a(4) =1158a(5) =6150a(6) =32656a(7) =173719a(8) =926664a(9) =4958556a(10) =26619438a(11) =143365880a(12) =774562478a(13) =4197344582a(14) =22810572062a(15) =124300860689a(16) =679081142350