a(n) = Sum_{k=2..n}(7^k).

Terms

a(0) =0a(1) =49a(2) =392a(3) =2793a(4) =19600a(5) =137249a(6) =960792a(7) =6725593a(8) =47079200a(9) =329554449a(10) =2306881192a(11) =16148168393a(12) =113037178800a(13) =791260251649