a(n) = A027082(n, 2n-2).

Terms

a(0) =1a(1) =5a(2) =14a(3) =39a(4) =108a(5) =299a(6) =828a(7) =2299a(8) =6401a(9) =17875a(10) =50062a(11) =140601a(12) =395933a(13) =1117737a(14) =3162812a(15) =8969267a(16) =25487555a(17) =72565339a(18) =206969662a(19) =591304797a(20) =1691991626a(21) =4848703909a(22) =13914119144a(23) =39980978277a(24) =115023510495a(25) =331303274449