List questions from examples-counterexamples

Published on 11 Apr 2026 - 17:07

83

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Can you give me an example of a function that is either upper OR lower quasi-continuous but not both?

Published on 16 Apr 2026 - 8:41

74

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Does $f \leq f \circ f^\dagger \circ f$ hold in an arbitrary allegory?

Published on 16 Apr 2026 - 3:08

275

Views

Upper half plane with irrational slope topology.

Published on 13 Apr 2026 - 2:22

999

Views

Homeomorphism between Space and Product: $X \cong X \times X$

Published on 15 Apr 2026 - 2:55

52

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$a\vee b$ and $a\wedge b$ have complements. Does $a$ have one?

Published on 15 Apr 2026 - 1:07

2.5k

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Necessity of completeness of the inner product space in Riesz representation theorem

Published on 14 Apr 2026 - 1:01

1.7k

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Nice convergent subsequence of $\cos(n)$.

Published on 13 Apr 2026 - 19:45

193

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Open covers of a topological space $X$

Published on 16 Apr 2026 - 12:52

630

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Example of non-noetherian algebras which are tensor products of noetherian algebras

Published on 14 Apr 2026 - 13:30

93

Views

Relation between positive integers greater than 2

Published on 14 Apr 2026 - 11:19

521

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About a counterexample for the Open Mapping Theorem

Published on 11 Apr 2026 - 15:29

1.3k

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Fibrations with isomorphic fibers, but not Zariski locally trivial

Published on 11 Apr 2026 - 4:27

1.4k

Views

My problem in understanding the minimal counterexample technique

Published on 29 Mar 2026 - 6:28

828

Views

Can we make $\mathbb{Z}$ into a field?

Published on 09 Apr 2026 - 13:04

List Question

Connected and Compact preserving function is not continuous example?

Can you give me an example of a function that is either upper OR lower quasi-continuous but not both?

Does $f \leq f \circ f^\dagger \circ f$ hold in an arbitrary allegory?

Upper half plane with irrational slope topology.

Homeomorphism between Space and Product: $X \cong X \times X$

$a\vee b$ and $a\wedge b$ have complements. Does $a$ have one?

Necessity of completeness of the inner product space in Riesz representation theorem

Nice convergent subsequence of $\cos(n)$.

Open covers of a topological space $X$

Example of non-noetherian algebras which are tensor products of noetherian algebras

Relation between positive integers greater than 2

About a counterexample for the Open Mapping Theorem

Fibrations with isomorphic fibers, but not Zariski locally trivial

My problem in understanding the minimal counterexample technique

Can we make $\mathbb{Z}$ into a field?

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