User David C. Ullrich - Mathematics Stack Exchange

82 votes

Are there theoretical applications of trigonometry?

answered May 2, 2016 at 0:35

63 votes

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Did Joseph Fourier ever make a pure mathematical mistake?

answered Jun 24, 2016 at 15:05

58 votes

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What is the average rational number?

answered May 28, 2016 at 14:51

50 votes

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Is every noninvertible matrix a zero divisor?

answered Jul 15, 2015 at 17:04

43 votes

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Problem of a slow thinker

answered Jul 14, 2018 at 15:09

41 votes

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Does every infinite field contain a countably infinite subfield?

answered Nov 7, 2015 at 19:00

32 votes

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Second derivative of $\det(\mathbb{1}+tA)$?

answered Jun 25, 2018 at 15:26

29 votes

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Reference for "distributional derivative being zero implies being constant"

answered Aug 15, 2015 at 0:22

28 votes

If $f$ is a smooth real valued function on real line such that $f'(0)=1$ and $|f^{(n)} (x)|$ is uniformly bounded by $1$ , then $f(x)=\sin x$?

answered Jul 19, 2016 at 15:12

25 votes

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If every five point subset of a metric space can be isometrically embedded in the plane then is it possible for the metric space also?

answered Jun 17, 2016 at 15:21

25 votes

Extension of real analytic function to a complex analytic function

answered Apr 27, 2016 at 14:22

22 votes

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Is every self-adjoint operator bounded?

answered Jan 12, 2018 at 16:50

21 votes

Can we recover a space from its continuous functions?

answered Oct 1, 2015 at 16:24

20 votes

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Proving that the tangent to a convex function is always below the function

answered Apr 27, 2016 at 21:54

20 votes

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Clarification in appendix 3 of Ulrich Complex Analysis

answered Jan 10, 2023 at 20:14

19 votes

Books on complex analysis (Ahlfors, Conway and Lang)

answered Aug 4, 2015 at 17:37

19 votes

A continuous, nowhere differentiable but invertible function?

answered Jul 16, 2018 at 17:37

18 votes

What are Some Tricks to Remember Fatou's Lemma?

answered Apr 23, 2016 at 23:27

17 votes

$\int_{0}^{x} f(t) dt \ge {f(x)}$ holds or not on $[0,1]$

answered Sep 5, 2015 at 16:49

17 votes

A field has only one isomorphic subfield to itself?

answered Mar 31, 2016 at 17:16

17 votes

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How to remember Stolz Angle correctly

answered Jul 16, 2016 at 16:53

16 votes

Every normed space has a completion?

answered May 2, 2016 at 12:55

16 votes

Understanding the philosophy behind the axiomatic definition of reals

answered Feb 18, 2018 at 16:44

16 votes

Questions about Aleph-Aleph-Null

answered Jan 2, 2019 at 15:45

15 votes

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Monotone functions and non-vanishing of derivative

answered Jul 1, 2016 at 15:54

15 votes

A Continuous Function with a Divergent Fourier Series

answered Jul 15, 2016 at 16:30

15 votes

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What does 2 to the power x mean in set theory

answered Dec 16, 2017 at 16:17

15 votes

Understanding successor sets

answered Jan 5, 2016 at 21:48

15 votes

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The 2-norm of the integral vs the integral of the 2-norm

answered Feb 15, 2016 at 15:07

15 votes

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Uniform limit of Lipschitz functions is a Lipschitz function

answered Feb 27, 2016 at 14:28

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2,642 Answers

Are there theoretical applications of trigonometry?

Did Joseph Fourier ever make a pure mathematical mistake?

What is the average rational number?

Is every noninvertible matrix a zero divisor?

Problem of a slow thinker

Does every infinite field contain a countably infinite subfield?

Second derivative of $\det(\mathbb{1}+tA)$?

Reference for "distributional derivative being zero implies being constant"

If $f$ is a smooth real valued function on real line such that $f'(0)=1$ and $|f^{(n)} (x)|$ is uniformly bounded by $1$ , then $f(x)=\sin x$?

If every five point subset of a metric space can be isometrically embedded in the plane then is it possible for the metric space also?

Extension of real analytic function to a complex analytic function

Is every self-adjoint operator bounded?

Can we recover a space from its continuous functions?

Proving that the tangent to a convex function is always below the function

Clarification in appendix 3 of Ulrich Complex Analysis

Books on complex analysis (Ahlfors, Conway and Lang)

A continuous, nowhere differentiable but invertible function?

What are Some Tricks to Remember Fatou's Lemma?

$\int_{0}^{x} f(t) dt \ge {f(x)}$ holds or not on $[0,1]$

A field has only one isomorphic subfield to itself?

How to remember Stolz Angle correctly

Every normed space has a completion?

Understanding the philosophy behind the axiomatic definition of reals

Questions about Aleph-Aleph-Null

Monotone functions and non-vanishing of derivative

A Continuous Function with a Divergent Fourier Series

What does 2 to the power x mean in set theory

Understanding successor sets

The 2-norm of the integral vs the integral of the 2-norm

Uniform limit of Lipschitz functions is a Lipschitz function