a(n) = (3*9^n - 5^n)/2.

Terms

a(0) =1a(1) =11a(2) =109a(3) =1031a(4) =9529a(5) =87011a(6) =789349a(7) =7135391a(8) =64374769a(9) =580154171a(10) =5225293789a(11) =47047175351a(12) =423522234409