a(n) = 2 + 4^n + 2*n*(n - 1).

Terms

a(0) =6a(1) =22a(2) =78a(3) =282a(4) =1066a(5) =4158a(6) =16470a(7) =65650a(8) =262290a(9) =1048758a(10) =4194526a(11) =16777482a(12) =67109178a(13) =268435822a(14) =1073742246a(15) =4294967778a(16) =17179869730a(17) =68719477350a(18) =274877907630