a(n) = (11*5^n-1)/2.

Terms

a(0) =5a(1) =27a(2) =137a(3) =687a(4) =3437a(5) =17187a(6) =85937a(7) =429687a(8) =2148437a(9) =10742187a(10) =53710937a(11) =268554687a(12) =1342773437a(13) =6713867187a(14) =33569335937a(15) =167846679687a(16) =839233398437