User Najib Idrissi

102 votes

How to put 9 pigs into 4 pens so that there are an odd number of pigs in each pen?

answered Nov 7, 2013 at 0:23

66 votes

Accepted

Why is $\text{Hom}(V,W)$ the same thing as $V^* \otimes W$?

answered Feb 17, 2014 at 15:38

46 votes

Accepted

The "magic diagram" is cartesian

answered May 2, 2014 at 12:26

46 votes

Accepted

Square of a second derivative is the fourth derivative

answered Nov 23, 2015 at 14:26

40 votes

Accepted

Can this be considered a "rigorous" definition of a limit?

answered Sep 13, 2016 at 14:51

40 votes

Accepted

Applications of Eckmann–Hilton argument

answered Mar 24, 2015 at 8:42

37 votes

If a topological space is homeomorphic to a smooth manifold, then will it be a smooth manifold?

answered Jan 22, 2018 at 14:09

34 votes

Accepted

Motivation for the mapping cone complexes

answered Feb 26, 2016 at 8:59

31 votes

If a nonempty set of real numbers is open and closed, is it $\mathbb{R}$? Why/Why not?

answered Apr 13, 2014 at 14:57

30 votes

Accepted

What is the motivation for semidirect products?

answered Feb 22, 2014 at 21:19

29 votes

Accepted

bijective homomorphisms between non isomorphic posets, example , explanation needed

answered Oct 20, 2014 at 11:25

29 votes

Accepted

Is the set of non invertible matrices simply connected? What are their homotopy and homology groups?

answered May 2, 2019 at 7:42

26 votes

Accepted

Rank-nullity theorem for free $\mathbb Z$-modules

answered Oct 31, 2014 at 13:15

26 votes

A fiber bundle over Euclidean space is trivial.

answered Dec 30, 2014 at 10:49

25 votes

If $f$ is continuous at $a$, is it continuous in some open interval around $a$?

answered Jun 2, 2013 at 20:46

25 votes

Is homology with coefficients in a field isomorphic to cohomology?

answered Mar 7, 2016 at 19:57

23 votes

Abelianization of free group is the free abelian group

answered Feb 6, 2014 at 15:11

22 votes

What are the irreducible representations of the cyclic group $C_n$ over a real vector space $V$?

answered Nov 8, 2015 at 8:03

21 votes

Accepted

If $\sum_{n = 1}^\infty {{a_n}}$ converges, then is $\sum_{n = 1}^\infty (1+a_n)^{-1}$ a convergent series?

answered Dec 3, 2013 at 21:50

21 votes

The loop space of the classifying space is the group: $\Omega(BG) \cong G$

answered Jul 13, 2013 at 17:14

21 votes

Accepted

Prove: The weak closure of the unit sphere is the unit ball.

answered Jun 4, 2012 at 19:56

21 votes

Accepted

Is there a onto group homomorphism from $\Bbb Q$ to $\Bbb Z$?

answered Dec 5, 2016 at 10:24

20 votes

Sum of $\cos(k x)$

answered May 31, 2012 at 13:51

20 votes

Accepted

$U^*\otimes V$ versus $L(U,V)$ for infinite dimensional spaces

answered Nov 19, 2013 at 16:56

19 votes

Accepted

Constant maps induce zero homomorphism

answered Sep 14, 2014 at 15:51

19 votes

Is a group homomorphism a module homomorphism?

answered Jul 12, 2016 at 8:30

18 votes

Is there an analytic function satisfying $\,\,f\big(\!\frac 1 n\!\big)=\frac 1 {\sqrt{n}}$ for all $n\in\mathbb N$?

answered Feb 18, 2014 at 8:38

18 votes

Accepted

All subgroups normal $\implies$ abelian group

answered May 13, 2016 at 7:24

17 votes

Are all good mathematicians fluent in computational aspects of mathematics

answered Dec 6, 2013 at 14:28

17 votes

Euler characteristic: dependence on coefficients

answered Jul 11, 2015 at 6:57

Stack Exchange Network

932 Answers

How to put 9 pigs into 4 pens so that there are an odd number of pigs in each pen?

Why is $\text{Hom}(V,W)$ the same thing as $V^* \otimes W$?

The "magic diagram" is cartesian

Square of a second derivative is the fourth derivative

Can this be considered a "rigorous" definition of a limit?

Applications of Eckmann–Hilton argument

If a topological space is homeomorphic to a smooth manifold, then will it be a smooth manifold?

Motivation for the mapping cone complexes

If a nonempty set of real numbers is open and closed, is it $\mathbb{R}$? Why/Why not?

What is the motivation for semidirect products?

bijective homomorphisms between non isomorphic posets, example , explanation needed

Is the set of non invertible matrices simply connected? What are their homotopy and homology groups?

Rank-nullity theorem for free $\mathbb Z$-modules

A fiber bundle over Euclidean space is trivial.

If $f$ is continuous at $a$, is it continuous in some open interval around $a$?

Is homology with coefficients in a field isomorphic to cohomology?

Abelianization of free group is the free abelian group

What are the irreducible representations of the cyclic group $C_n$ over a real vector space $V$?

If $\sum_{n = 1}^\infty {{a_n}}$ converges, then is $\sum_{n = 1}^\infty (1+a_n)^{-1}$ a convergent series?

The loop space of the classifying space is the group: $\Omega(BG) \cong G$

Prove: The weak closure of the unit sphere is the unit ball.

Is there a onto group homomorphism from $\Bbb Q$ to $\Bbb Z$?

Sum of $\cos(k x)$

$U^*\otimes V$ versus $L(U,V)$ for infinite dimensional spaces

Constant maps induce zero homomorphism

Is a group homomorphism a module homomorphism?

Is there an analytic function satisfying $\,\,f\big(\!\frac 1 n\!\big)=\frac 1 {\sqrt{n}}$ for all $n\in\mathbb N$?

All subgroups normal $\implies$ abelian group

Are all good mathematicians fluent in computational aspects of mathematics

Euler characteristic: dependence on coefficients