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MathNet 2006 — Part 02 Question 153

Math Olympiad Vault · MathNet 2006 — Part 02

Question 153

Professor Piraldo takes part in soccer matches with a lot of goals and judges a match in his own peculiar way. A match with score of $m$ goals to $n$ goals, $m \ge n$, is *tough* when $m \le f(n)$, where $f(n)$ is defined by $f(0) = 0$ and, for $n \ge 1$, $f(n) = 2n - f(r) + r$, where $r$ is the largest integer such that $r \lt n$ and $f(r) \le n$.
Let $\phi = \frac{1+\sqrt{5}}{2}$. Prove that a match with score of $m$ goals to $n$, $m \ge n$, is tough if $m \le \phi n$ and is not tough if $m \ge \phi n + 1$.