a(n) = sigma(6^(n-1)).

Terms

a(0) =1a(1) =12a(2) =91a(3) =600a(4) =3751a(5) =22932a(6) =138811a(7) =836400a(8) =5028751a(9) =30203052a(10) =181308931a(11) =1088123400a(12) =6529545751a(13) =39179682372a(14) =235085301451