a(n) = 7^n - n^2.

Terms

a(0) =1a(1) =6a(2) =45a(3) =334a(4) =2385a(5) =16782a(6) =117613a(7) =823494a(8) =5764737a(9) =40353526a(10) =282475149a(11) =1977326622a(12) =13841287057a(13) =96889010238a(14) =678223072653