List questions from examples-counterexamples

Published on 12 Apr 2026 - 17:12

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Almost-Cantor Set Example

Published on 08 Apr 2026 - 10:50

335

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Deducing measurability of $f$ and $g$ from the measurability of $f+g$?

Published on 15 Apr 2026 - 0:58

234

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Does $\operatorname{int}(f^{-1}[A]) \subseteq f^{−1} [\operatorname{int}(A)]$ hold for continuous $f$?

Published on 14 Apr 2026 - 6:16

186

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Counter-example of Necas inequality

Published on 11 Apr 2026 - 15:06

55

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Integer solution to linear system

Published on 10 Apr 2026 - 8:07

561

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$\matrix {AB}=\matrix 0$ and $\matrix A\not= \matrix 0$, then $|\matrix B|=0$ ?!

Published on 14 Apr 2026 - 2:53

99

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Spherical coordinates to compute a definite integral

Published on 13 Apr 2026 - 16:38

1.6k

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Which insights are particularly simple to get through category theory?

Published on 11 Apr 2026 - 5:06

254

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Two spaces, each a retract of the other, which are not homotopy equivalent

Published on 12 Apr 2026 - 18:48

998

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Example of a monoid with two distinct left inverses

Published on 13 Apr 2026 - 21:20

359

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Double annihilator for subspaces of $E^*$: counter example.

Published on 12 Apr 2026 - 8:33

129

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Sub-group of a free group that is characteristic But not totally characteristic

Published on 26 Mar 2026 - 20:16

1.4k

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A principal ideal is maximal in $\mathbb Z[x]$?

Published on 11 Apr 2026 - 5:16

241

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An ideal is maximal iff it is generated by an irreducible elemnt

Published on 12 Apr 2026 - 16:43

List Question

Counter example to the fundamental lemma of calculus of variations

Almost-Cantor Set Example

Deducing measurability of $f$ and $g$ from the measurability of $f+g$?

Does $\operatorname{int}(f^{-1}[A]) \subseteq f^{−1} [\operatorname{int}(A)]$ hold for continuous $f$?

Counter-example of Necas inequality

Integer solution to linear system

$\matrix {AB}=\matrix 0$ and $\matrix A\not= \matrix 0$, then $|\matrix B|=0$ ?!

Spherical coordinates to compute a definite integral

Which insights are particularly simple to get through category theory?

Two spaces, each a retract of the other, which are not homotopy equivalent

Example of a monoid with two distinct left inverses

Double annihilator for subspaces of $E^*$: counter example.

Sub-group of a free group that is characteristic But not totally characteristic

A principal ideal is maximal in $\mathbb Z[x]$?

An ideal is maximal iff it is generated by an irreducible elemnt

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