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IMO 2006 — Complete Shortlist Question 1

IMO Shortlist · IMO 2006 — Complete Shortlist

Question 1

A sequence of real numbers $a_{0}, a_{1}, a_{2}, \ldots$ is defined by the formula
$$
a_{i+1}=\left\lfloor a_{i}\right\rfloor \cdot\left\langle a_{i}\right\rangle \quad \text{ for } \quad i \geq 0
$$
here $a_{0}$ is an arbitrary real number, $\left\lfloor a_{i}\right\rfloor$ denotes the greatest integer not exceeding $a_{i}$, and $\left\langle a_{i}\right\rangle=a_{i}-\left\lfloor a_{i}\right\rfloor$. Prove that $a_{i}=a_{i+2}$ for $i$ sufficiently large.