a(n) = 5^n + 1.

Terms

a(0) =2a(1) =6a(2) =26a(3) =126a(4) =626a(5) =3126a(6) =15626a(7) =78126a(8) =390626a(9) =1953126a(10) =9765626a(11) =48828126a(12) =244140626a(13) =1220703126a(14) =6103515626a(15) =30517578126a(16) =152587890626a(17) =762939453126