a(n) = A026615(2*n-1, n-2).

Terms

a(0) =1a(1) =9a(2) =37a(3) =147a(4) =576a(5) =2244a(6) =8723a(7) =33891a(8) =131716a(9) =512278a(10) =1994202a(11) =7770734a(12) =30310320a(13) =118343970a(14) =462501135a(15) =1809134115a(16) =7082699580a(17) =27750808470a(18) =108812919270a(19) =426966196410