T(2n,n+1), T given by A026758.

Terms

a(0) =1a(1) =4a(2) =16a(3) =65a(4) =268a(5) =1120a(6) =4738a(7) =20265a(8) =87542a(9) =381600a(10) =1677170a(11) =7427098a(12) =33118073a(13) =148618605a(14) =670849720a(15) =3044548675a(16) =13886260129a(17) =63627798355a(18) =292789942333