a(n) = 4^n - 4*n.

Terms

a(0) =1a(1) =0a(2) =8a(3) =52a(4) =240a(5) =1004a(6) =4072a(7) =16356a(8) =65504a(9) =262108a(10) =1048536a(11) =4194260a(12) =16777168a(13) =67108812a(14) =268435400a(15) =1073741764a(16) =4294967232a(17) =17179869116a(18) =68719476664a(19) =274877906868