a(n) = (10^n + 4^n)/2.

Terms

a(0) =1a(1) =7a(2) =58a(3) =532a(4) =5128a(5) =50512a(6) =502048a(7) =5008192a(8) =50032768a(9) =500131072a(10) =5000524288a(11) =50002097152a(12) =500008388608