Counterexample
A counterexample is a form of counter proof.
Given a hypothesis stating that F(x) is true for all x in S, show that there exists a b in S such that F(b) is false, contradicting the hypothesis.
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ProofThis entry contributed by Wiktor K. Macura
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Macura, Wiktor K. "Counterexample." From MathWorld--A Wolfram Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/Counterexample.html