a(n) = A027113(n, 2n-3).

Terms

a(0) =1a(1) =4a(2) =13a(3) =40a(4) =120a(5) =352a(6) =1022a(7) =2948a(8) =8473a(9) =24304a(10) =69649a(11) =199544a(12) =571795a(13) =1639244a(14) =4702499a(15) =13500396a(16) =38790727a(17) =111555296a(18) =321100627a(19) =925086012a(20) =2667533646a(21) =7698716068a(22) =22238169212a(23) =64289860528a(24) =186010834243a(25) =538611262492