a(n) = T(2n+1,n+4), T given by A026725.

Terms

a(0) =1a(1) =9a(2) =56a(3) =300a(4) =1487a(5) =7041a(6) =32381a(7) =146017a(8) =649395a(9) =2859231a(10) =12494914a(11) =54291912a(12) =234860677a(13) =1012433965a(14) =4352210327a(15) =18666918033a(16) =79916230409a(17) =341615895659