T(2n,n-1), T given by A026714.

Terms

a(0) =1a(1) =7a(2) =40a(3) =217a(4) =1157a(5) =6128a(6) =32366a(7) =170731a(8) =900070a(9) =4743641a(10) =24996639a(11) =131709418a(12) =693958384a(13) =3656283198a(14) =19263738121a(15) =101493477133a(16) =534729209960