T(2n-1,n-1), T given by A026681.

Terms

a(0) =1a(1) =5a(2) =17a(3) =87a(4) =324a(5) =1707a(6) =6595a(7) =35300a(8) =139114a(9) =751901a(10) =3000177a(11) =16322561a(12) =65684626a(13) =359049028a(14) =1453825024a(15) =7975236698a(16) =32444454286a(17) =178473218093a(18) =728737293093