List questions from examples-counterexamples

Published on 13 Apr 2026 - 12:40

64

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If $f_x(x,y)>0$, $f_{xx}(x,y)<0$, $f_y(x,y)>0$, $f_{yy}(x,y)<0$ can $f_{xy} $ change sign?

Published on 17 Apr 2026 - 13:18

203

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Does convexity of $f(x)$ imply convexity of $f(e^x)$?

Published on 14 Apr 2026 - 18:30

214

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Examples of functors which have both adjoints

Published on 12 Apr 2026 - 8:40

129

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The nonexistence of any "good" norm on $\ell^\infty(\mathbb R)$

Published on 13 Apr 2026 - 18:06

318

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Which operators commute with curl?

Published on 08 Apr 2026 - 17:55

169

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Sum of preimages of subspaces

Published on 08 Apr 2026 - 20:10

656

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Is an ideal finitely generated if its radical is finitely generated?

Published on 08 Apr 2026 - 17:02

48

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Reducing the Exponent of an $\ell^p$ Sequence / Counterexample

Published on 16 Apr 2026 - 1:40

61

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Example of a striclty convex real valued function that's discontinuous

Published on 14 Apr 2026 - 16:50

157

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What is the minimum number of broken queens required to cover an $n\times n$ board?

Published on 13 Apr 2026 - 18:28

96

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adjunctions from either the category of small categories or the category of groupoids

Published on 24 Apr 2026 - 21:57

34

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Find an example of its sequence

Published on 14 Apr 2026 - 9:26

188

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Counterexample? Equality holds in the Cauchy-Schwarz Inequality iff $\textbf{x} = \alpha\textbf{y}$ for some $\alpha \in \mathbb{R}$

Published on 14 Apr 2026 - 12:35

147

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Example of a map from the natural numbers to the positive rational numbers less than 1

Published on 14 Apr 2026 - 11:52

List Question

multiplier algebra and intersection

If $f_x(x,y)>0$, $f_{xx}(x,y)<0$, $f_y(x,y)>0$, $f_{yy}(x,y)<0$ can $f_{xy} $ change sign?

Does convexity of $f(x)$ imply convexity of $f(e^x)$?

Examples of functors which have both adjoints

The nonexistence of any "good" norm on $\ell^\infty(\mathbb R)$

Which operators commute with curl?

Sum of preimages of subspaces

Is an ideal finitely generated if its radical is finitely generated?

Reducing the Exponent of an $\ell^p$ Sequence / Counterexample

Example of a striclty convex real valued function that's discontinuous

What is the minimum number of broken queens required to cover an $n\times n$ board?

adjunctions from either the category of small categories or the category of groupoids

Find an example of its sequence

Counterexample? Equality holds in the Cauchy-Schwarz Inequality iff $\textbf{x} = \alpha\textbf{y}$ for some $\alpha \in \mathbb{R}$

Example of a map from the natural numbers to the positive rational numbers less than 1

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