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mathcounterexamples.net
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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!

My LinkedIn page. A long time ago I programmed some modules of CATIA a famous computed aided design solution.

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question
30
Group $G$ whose center $Z(G)$ is cyclic and with $G/Z(G)$ commutative
Jun 28, 2015
answer
23
Proof: Is there a line in the xy plane that goes through only rational coordinates?
Dec 12, 2015
answer
20
Proof that the number of proofs is countably infinite
Jun 11, 2018
answer
18
How to check if a function is convex
Aug 16, 2019
answer
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, 2018
answer
18
$ \bigcup_{n=1}^{\infty}\{z\in\Bbb C:z^n=1\}=\{z\in\Bbb C:|z|=1\}$?
Feb 1, 2019
question
17
$a_n =0$ for all $n \in \mathbb N$ if $\sum a_n^k =0$ for all integers $k \ge 1$
Sep 13, 2015
answer
17
Differentiability of a two variable function $f(x,y)=\dfrac{1}{1+x-y}$
Aug 22, 2015
answer
16
A+B is closed if one of them is compact
Sep 5, 2015
answer
16
Convergence of infinite sum (factorial and root)
Nov 22, 2016
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67
Results true in a dimension and false for higher dimensions
36
Why does this sequence converges to $\pi$?
18
What are your favorite instructional counterexamples on sequences?
14
ODE properties true in finite dimension but not in Banach spaces of infinite dimension
10
Probability over a plane