a(n) = ceiling(4^n/n).

Terms

a(0) =4a(1) =8a(2) =22a(3) =64a(4) =205a(5) =683a(6) =2341a(7) =8192a(8) =29128a(9) =104858a(10) =381301a(11) =1398102a(12) =5162221a(13) =19173962a(14) =71582789a(15) =268435456a(16) =1010580541a(17) =3817748708a(18) =14467258261a(19) =54975581389a(20) =209430786244a(21) =799644820201