a(n) = T(2n,n-2), T given by A026670.

Terms

a(0) =1a(1) =7a(2) =37a(3) =177a(4) =808a(5) =3596a(6) =15764a(7) =68446a(8) =295294a(9) =1268356a(10) =5430734a(11) =23199304a(12) =98933705a(13) =421352919a(14) =1792709561a(15) =7621345733a(16) =32380443643a(17) =137504761035a(18) =583684770103