Inverse Weigh transform of 4^n.

Terms

a(0) =4a(1) =10a(2) =20a(3) =70a(4) =204a(5) =690a(6) =2340a(7) =8230a(8) =29120a(9) =104958a(10) =381300a(11) =1398430a(12) =5162220a(13) =19175130a(14) =71582716a(15) =268439590a(16) =1010580540a(17) =3817763040a(18) =14467258260a(19) =54975633906a(20) =209430785460a(21) =799645010850