a(n) = (13^n + 1)/2.

Terms

a(0) =1a(1) =7a(2) =85a(3) =1099a(4) =14281a(5) =185647a(6) =2413405a(7) =31374259a(8) =407865361a(9) =5302249687a(10) =68929245925a(11) =896080197019