a(n) = 2*5^n - 3.

Terms

a(0) =7a(1) =47a(2) =247a(3) =1247a(4) =6247a(5) =31247a(6) =156247a(7) =781247a(8) =3906247a(9) =19531247a(10) =97656247a(11) =488281247a(12) =2441406247a(13) =12207031247a(14) =61035156247a(15) =305175781247