a(n) = Sum_{i=3^n..3^(n+1)-1} i.

Terms

a(0) =3a(1) =33a(2) =315a(3) =2889a(4) =26163a(5) =235953a(6) =2125035a(7) =19129689a(8) =172180323a(9) =1549662273a(10) =13947078555a(11) =125524061289