a(n) = T(2n-1,n-1), T given by A026703.

Terms

a(0) =1a(1) =5a(2) =24a(3) =116a(4) =564a(5) =2752a(6) =13457a(7) =65891a(8) =322905a(9) =1583317a(10) =7766507a(11) =38106295a(12) =187002147a(13) =917808733a(14) =4505028968a(15) =22114223996a(16) =108559218028a(17) =532938398090