What we need to find out the periods of a periodic difference equation.

We study the set of periods of a p-periodic nonautonomous difference equation xn+1 = fn(xn), where fn: I → I are continuous interval maps satisfying the generic condition: Card{x ∈ I: fi(x) = fj(x)} < ∞, for all i ≠ j(mod p). [ABSTRACT FROM AUTHOR]