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MathNet 2011 Question 91

Math Olympiad Vault · MathNet 2011

Question 91

Consider an increasing continuous function $f : [0, 1] \to \mathbb{R}$ and define the sequence $(a_n)_{n \ge 1}$ by $a_n = \frac{1}{2^n} \sum_{k=1}^{2^n} f(\frac{k}{2^n})$, for all integers $n \ge 1$.

a) Prove that the sequence $(a_n)_{n \ge 1}$ is increasing.

b) Given that there exists $p \in \mathbb{N}^*$ such that $a_p = \int_0^1 f(x)dx$, prove that $f$ is a constant function.