Hello, I think this type of recurrence is kind of easy to solve explicitely, a(n) = (-3/17*17^(1/2)+4)*(3+17^(1/2))^n+(3/17*17^(1/2)+4)*(3-17^(1/2))^n I obtained that formula from GFUN: listtorec, then rsolve. Simon Plouffe