a(n) = (2^n + 1)^3 - 2.

Terms

a(0) =6a(1) =25a(2) =123a(3) =727a(4) =4911a(5) =35935a(6) =274623a(7) =2146687a(8) =16974591a(9) =135005695a(10) =1076890623a(11) =8602523647a(12) =68769820671a(13) =549957165055