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

Terms

a(0) =1a(1) =8a(2) =24a(3) =76a(4) =272a(5) =1044a(6) =4120a(7) =16412a(8) =65568a(9) =262180a(10) =1048616a(11) =4194348a(12) =16777264a(13) =67108916a(14) =268435512a(15) =1073741884a(16) =4294967360a(17) =17179869252a(18) =68719476808a(19) =274877907020