olymmath

CIIM 2009 Problem 5

  • Day 2

Let f:R→Rf:\mathbb{R} \to \mathbb{R} , such that

i) For all a∈Ra \in \mathbb{R} and all ϵ>0\epsilon > 0 , exists δ>0\delta > 0 such that ∣x−a∣<δ⇒f(x)<f(a)+ϵ.|x-a| < \delta \Rightarrow f(x) < f(a) + \epsilon. ii) For all b∈Rb\in \mathbb{R} and all ϵ>0\epsilon > 0 , exists x,y∈Rx,y \in \mathbb{R} with b−ϵ<x<b<y<b+ϵb - \epsilon < x < b < y < b + \epsilon , such that ∣f(x)−f(b)∣<ϵ|f(x)-f(b)|< \epsilon and ∣f(y)−f(b)∣<ϵ.|f(y)-f(b)| < \epsilon. Prove that if f(a)<d<f(d)f(a) < d < f(d) there exists cc with a<c<ba < c < b or b<c<ab < c < a such that f(c)=df(c) = d .

Dataset text AI-MO AoPS. Quoted with source.

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