a(n) = 7*a(n-1) + 2*7^n - 1, a(0) = 0.

Terms

a(0) =0a(1) =13a(2) =188a(3) =2001a(4) =18808a(5) =165269a(6) =1392180a(7) =11392345a(8) =91276016a(9) =719639325a(10) =5602425772a(11) =43171633889a(12) =329884011624