a(n) = 4*a(n-1) + 1, a(1)=7.

Terms

a(0) =7a(1) =29a(2) =117a(3) =469a(4) =1877a(5) =7509a(6) =30037a(7) =120149a(8) =480597a(9) =1922389a(10) =7689557a(11) =30758229a(12) =123032917a(13) =492131669a(14) =1968526677a(15) =7874106709a(16) =31496426837a(17) =125985707349a(18) =503942829397