a(n) = floor((1 + 4^n)/(1 + 4*n)).

Terms

a(0) =1a(1) =1a(2) =5a(3) =15a(4) =48a(5) =163a(6) =565a(7) =1985a(8) =7085a(9) =25575a(10) =93206a(11) =342392a(12) =1266205a(13) =4709393a(14) =17602325a(15) =66076419a(16) =248983611a(17) =941362695a(18) =3569842947a(19) =13574217626a(20) =51741723660a(21) =197665011735a(22) =756653163200