Fubini's Theorem contradiction
132701
Contradicting Fubini's theorem
134575
Show that the mapping $f → f~'$ from $C^1([0 , 1])$ to $C([0 , 1])$ is not continuous.
32524
Flawed proof that all positive integers are equal
105233
Is there a counterexample for the claim: if $A \oplus B\cong A\oplus C$ then $B\cong C$?
58388
Counterexamples to the continuum hypothesis
100399
A doubt with a part of a certain proof.
132207
What's behind the Banach-Tarski paradox?
38221
What is wrong with this proof there is no $\omega$-th worldly cardinal
43626
Open Mapping Theorem: counterexample
53123
Trying to derive a contradiction with this simple inequality,
120406
$f$ strictly increasing does not imply $f'>0$
35751
Is There a Problem with This Professor's Proof Concerning Interior and Boundary Points?
106799
Proof Verification: Converse of Intermediate Value Theorem
57969
Prove that $0$ is not an essential singularity (UW Madison Qualifying exam)
35370
Why is this proposition provable and its converse not?
153532
Proving $a^2$ is even $\implies$ $a$ is even by contradiction
63952
Disproving: If $(\Gamma\vdash(\phi\,\lor\psi))$, either $(\Gamma\vdash\phi)$ or $(\Gamma\vdash\psi)$ is true