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

Terms

a(0) =4a(1) =8a(2) =21a(3) =64a(4) =204a(5) =682a(6) =2340a(7) =8192a(8) =29127a(9) =104857a(10) =381300a(11) =1398101a(12) =5162220a(13) =19173961a(14) =71582788a(15) =268435456a(16) =1010580540a(17) =3817748707a(18) =14467258260a(19) =54975581388a(20) =209430786243a(21) =799644820200