a(n) = (2*n + 1)*7^n.

Terms

a(0) =1a(1) =21a(2) =245a(3) =2401a(4) =21609a(5) =184877a(6) =1529437a(7) =12353145a(8) =98001617a(9) =766718533a(10) =5931980229a(11) =45478515089a(12) =346032180025