(11*5^n+1)/2.

Terms

a(0) =6a(1) =28a(2) =138a(3) =688a(4) =3438a(5) =17188a(6) =85938a(7) =429688a(8) =2148438a(9) =10742188a(10) =53710938a(11) =268554688a(12) =1342773438a(13) =6713867188a(14) =33569335938a(15) =167846679688a(16) =839233398438