User Elias Costa

44 votes

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How can I prove that $xy\leq x^2+y^2$?

answered Apr 10, 2013 at 16:31

39 votes

How does a non-mathematician go about publishing a proof in a way that ensures it to be up to the mathematical community's standards?

answered Jan 10, 2013 at 16:05

25 votes

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Why the $\nabla f(x)$ in the direction orthogonal to $f(x)$?

answered May 25, 2013 at 14:01

21 votes

Can't argue with success? Looking for "bad math" that "gets away with it"

answered Dec 18, 2012 at 16:55

15 votes

Funny identities

answered Jan 13, 2013 at 0:25

14 votes

The Basel problem

answered Jan 16, 2013 at 15:33

9 votes

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how to prove $(-1)\cdot(-1)=1$ based only on the field axioms?

answered Oct 25, 2013 at 15:52

9 votes

Nonzero $f \in C([0, 1])$ for which $\int_0^1 f(x)x^n dx = 0$ for all $n$

answered Dec 20, 2011 at 12:38

9 votes

Solve equations $\sqrt{t +9} - \sqrt{t} = 1$

answered May 22, 2013 at 13:02

8 votes

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Proving that Mahalanobis norm is a norm indeed

answered Jan 12, 2018 at 19:58

8 votes

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Some questions about Hilbert matrix

answered Feb 22, 2012 at 19:46

8 votes

Free online mathematical software

answered May 14, 2013 at 16:46

7 votes

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Real analysis book suggestion

answered Dec 1, 2014 at 10:59

7 votes

The inverse of a bijective holomorphic function is also holomorphic

answered Dec 2, 2014 at 14:09

7 votes

What Mathematics questions can be better solved with concepts from Physics?

answered Dec 10, 2012 at 13:31

6 votes

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$ \sum_{n=1}^{\infty}a_{n} $ diverges but $ \sum_{n=1}^{\infty}\frac{a_{n}}{1+a_{n}^{2}} $ sometimes converges and sometime diverges.

answered Jan 31, 2015 at 18:07

6 votes

Why is $\sin(x) = \sin(180^{\circ}-x)$

answered Nov 16, 2013 at 12:43

6 votes

Accepted

Special formula for the permanent of the sum of two matrices

answered Jan 22, 2018 at 21:41

6 votes

What distinguishes measure theory and probability theory?

answered Jan 4, 2021 at 18:39

6 votes

Proof of: If $P(A) = P(B) = 1$ then $P(A \cap B) = 1$.

answered Jan 23, 2019 at 15:41

6 votes

Prove that $\forall \epsilon > 0$: $\lim_{t\to\infty}t^{-2}\int_{0}^{t}[(f(x))^{1+\epsilon}/f'(x)]\,\mathrm dx =+\infty$

answered May 26, 2019 at 14:57

6 votes

Can't argue with success? Looking for "bad math" that "gets away with it"

answered Dec 18, 2012 at 17:18

6 votes

Proofs that involve Tricks

answered Dec 16, 2012 at 23:09

6 votes

The Meaning of the Fundamental Theorem of Calculus

answered Mar 20, 2013 at 14:14

5 votes

Solve the equation $z^3=z+\overline{z}$

answered Jul 9, 2013 at 22:08

5 votes

Simplest or nicest proof that $1+x \le e^x$

answered Sep 25, 2013 at 14:08

5 votes

Can the Gauss-Bonnet theorem be proven from Stokes's theorem?

answered Feb 1, 2019 at 20:45

5 votes

On Ph.D. Qualifying Exams

answered Sep 28, 2020 at 10:29

5 votes

Accepted

Matrix Geometric Series

answered Sep 13, 2018 at 11:49

5 votes

Finding circumference without using $\pi$

answered Jan 17, 2013 at 0:34

Stack Exchange Network

458 Answers

How can I prove that $xy\leq x^2+y^2$?

How does a non-mathematician go about publishing a proof in a way that ensures it to be up to the mathematical community's standards?

Why the $\nabla f(x)$ in the direction orthogonal to $f(x)$?

Can't argue with success? Looking for "bad math" that "gets away with it"

Funny identities

The Basel problem

how to prove $(-1)\cdot(-1)=1$ based only on the field axioms?

Nonzero $f \in C([0, 1])$ for which $\int_0^1 f(x)x^n dx = 0$ for all $n$

Solve equations $\sqrt{t +9} - \sqrt{t} = 1$

Proving that Mahalanobis norm is a norm indeed

Some questions about Hilbert matrix

Free online mathematical software

Real analysis book suggestion

The inverse of a bijective holomorphic function is also holomorphic

What Mathematics questions can be better solved with concepts from Physics?

$ \sum_{n=1}^{\infty}a_{n} $ diverges but $ \sum_{n=1}^{\infty}\frac{a_{n}}{1+a_{n}^{2}} $ sometimes converges and sometime diverges.

Why is $\sin(x) = \sin(180^{\circ}-x)$

Special formula for the permanent of the sum of two matrices

What distinguishes measure theory and probability theory?

Proof of: If $P(A) = P(B) = 1$ then $P(A \cap B) = 1$.

Prove that $\forall \epsilon > 0$: $\lim_{t\to\infty}t^{-2}\int_{0}^{t}[(f(x))^{1+\epsilon}/f'(x)]\,\mathrm dx =+\infty$

Can't argue with success? Looking for "bad math" that "gets away with it"

Proofs that involve Tricks

The Meaning of the Fundamental Theorem of Calculus

Solve the equation $z^3=z+\overline{z}$

Simplest or nicest proof that $1+x \le e^x$

Can the Gauss-Bonnet theorem be proven from Stokes's theorem?

On Ph.D. Qualifying Exams

Matrix Geometric Series

Finding circumference without using $\pi$