a(n) = (11^n + 1)/2.

Terms

a(0) =1a(1) =6a(2) =61a(3) =666a(4) =7321a(5) =80526a(6) =885781a(7) =9743586a(8) =107179441a(9) =1178973846a(10) =12968712301a(11) =142655835306