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mathcounterexamples.net

I'm living in Paris, France and doing Math as a hobby, with special interest in Mathematical logic and Analysis.

I opened in July 2014 a website dedicated to Math Counterexamples. I welcome any suggestion for additional counterexamples... at least at a level that I can understand!

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29 Group $G$ whose center $Z(G)$ is cyclic and with $G/Z(G)$ commutative Jun 28 '15
23 Proof: Is there a line in the xy plane that goes through only rational coordinates? Dec 12 '15
20 Proof that the number of proofs is countably infinite Jun 11 '18
18 Is $F=\left\lbrace f: f\in C_{\left[0, 1\right]}, f\left(0\right)=f\left(1\right)\right\rbrace$ complete metric space? Jun 28 '18
18 $ \bigcup_{n=1}^{\infty}\{z\in\Bbb C:z^n=1\}=\{z\in\Bbb C:|z|=1\}$? Feb 1 '19
16 Convergence of infinite sum (factorial and root) Nov 22 '16
14 L'Hôpital's rule: number of iterations… May 26 '18
13 Find $\lim\limits_{x\to \infty} \frac{x\sin x}{1+x^2}$ Jun 10 '15
13 Differentiability of a two variable function $f(x,y)=\dfrac{1}{1+x-y}$ Aug 22 '15
12 Having difficulty understanding what happens when reversing order of quaternion multiplication. Mar 12 '20
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