a(n) = 2^(2*n) - 2*n.

Terms

a(0) =1a(1) =2a(2) =12a(3) =58a(4) =248a(5) =1014a(6) =4084a(7) =16370a(8) =65520a(9) =262126a(10) =1048556a(11) =4194282a(12) =16777192a(13) =67108838a(14) =268435428a(15) =1073741794a(16) =4294967264a(17) =17179869150a(18) =68719476700a(19) =274877906906