a(n) = floor((1 + 4^n)/(1 + 2*n)).

Terms

a(0) =1a(1) =3a(2) =9a(3) =28a(4) =93a(5) =315a(6) =1092a(7) =3855a(8) =13797a(9) =49932a(10) =182361a(11) =671088a(12) =2485513a(13) =9256395a(14) =34636833a(15) =130150524a(16) =490853405a(17) =1857283155a(18) =7048151460a(19) =26817356775a(20) =102280151421a(21) =390937467653