a(n) = 5^n - 4.

Terms

a(0) =1a(1) =21a(2) =121a(3) =621a(4) =3121a(5) =15621a(6) =78121a(7) =390621a(8) =1953121a(9) =9765621a(10) =48828121a(11) =244140621a(12) =1220703121a(13) =6103515621a(14) =30517578121a(15) =152587890621a(16) =762939453121