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