A Class of Stationary Sequences

We define a class of sequences $a_n$ by $a_1=a$ and $a_{n+1}=P(a_n)$, where $P(x)$ is a polynomial with real coefficients. We then find out for which values $a$ and for which polynomials $P(x)$ these sequences will be constant after a certain rank. Then we generalize it from polynomials $P(x)$ to real functions $f(x)$.