a(n) = 2^(2*n+1) - C(2*n+3,n+1) + C(2*n+1,n).

Terms

a(0) =0a(1) =1a(2) =7a(3) =37a(4) =176a(5) =794a(6) =3473a(7) =14893a(8) =63004a(9) =263950a(10) =1097790a(11) =4540386a(12) =18696432a(13) =76717268a(14) =313889477a(15) =1281220733a(16) =5219170052a(17) =21224674118a(18) =86188320962a(19) =349550141078