a(n) = 8^n - 3*2^n + 5.

Terms

a(0) =7a(1) =57a(2) =493a(3) =4053a(4) =32677a(5) =261957a(6) =2096773a(7) =16776453a(8) =134216197a(9) =1073738757a(10) =8589928453a(11) =68719464453a(12) =549755789317