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Epsilon Principle

Last updated Nov 1, 2022

# Statement

Let a,bRa,b \in \mathbb{R}. Suppose for all ϵ>0\epsilon > 0, we have that abϵa \geq b - \epsilon. Then aba \geq b.

# Proof

Suppose not. Then a<ba < b and there exists some δ>0\delta > 0 s.t. a=bδa = b - \delta. But then a<bδ2a < b - \frac{\delta}{2} which contradicts

our assumption that abϵa \geq b - \epsilon for all ϵ>0\epsilon > 0. \blacksquare

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