a(n) = (21^n - 1)/20.

Terms

a(0) =0a(1) =1a(2) =22a(3) =463a(4) =9724a(5) =204205a(6) =4288306a(7) =90054427a(8) =1891142968a(9) =39714002329a(10) =833994048910