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139 Answers

Votes Activity Newest
57
Two metrics induce the same topology, but one is complete and the other isn't
Oct 23 '10 at 11:56
38
Idempotents in a local ring
Mar 24 '14 at 19:56
27
Irreducibility of $f(x)=x^4+3x^3-9x^2+7x+27$
Dec 18 '15 at 14:15
27
How to understand compactness?
Dec 25 '10 at 6:52
16
Is addition continuous?
Sep 28 '10 at 16:47
14
All homomorphisms from the ring $\mathbb Z \times \mathbb Z \to \mathbb Z$
Nov 15 '10 at 11:52
13
Combinatorial Proof of $\binom{\binom{n}{2}}{2} = 3 \binom{n}{3}+ 3 \binom{n}{4}$ for $n \geq 4$
Dec 25 '10 at 18:50
12
$\frac{\prod \mathbb{Z_p}}{\bigoplus \mathbb{Z_p}}$ is a divisible abelian group
Nov 12 '10 at 13:15
12
Proof - Square Matrix has maximal rank if and only if it is invertible
Feb 14 '11 at 12:32
12
Ahlfors's proof of analytic functions reducing to a constant
Feb 5 '11 at 7:36
12
Regular but not normal space
Jan 19 '11 at 16:19
11
Do all infinite subsets of $\mathbb N$ have a bijective function to $\mathbb N$?
Jan 29 '11 at 11:18
10
Why do elementary matrix operations not affect the row space of a given matrix?
Feb 3 '11 at 6:58
10
Show that there's a bijective $f: \mathbb{N} \to \mathbb{N}$ such that $\sum_{n=1}^{\infty} (-1)^{f(n)}\ln\frac{f(n)+1}{f(n)}=\ln 2010$
Jan 7 '11 at 14:39
9
If a matrix is invertible, is its multiplication commutative?
Feb 11 '11 at 7:01
7
Finite field extension with no non-trivial subextension
Feb 2 '17 at 10:54
7
Identity involving Euler's totient function: $\sum \limits_{k=1}^n \left\lfloor \frac{n}{k} \right\rfloor \varphi(k) = \frac{n(n+1)}{2}$
Oct 27 '10 at 6:25
7
$f(1-f(x))=f(x)$
Jan 27 '11 at 23:33
7
Why is the tensor product important when we already have direct and semidirect products?
Jan 25 '11 at 12:15
7
$(-1)\otimes (-1) \cong I$
Jan 15 '11 at 12:29
7
If $\int_0^x f \ dm$ is zero everywhere then $f$ is zero almost everywhere
Jan 3 '11 at 17:30
7
Ideals of subrings of $M_2(\mathbb{Q})$
Nov 28 '10 at 6:50
6
The fractional part of $n\log(n)$
Jan 23 '17 at 12:55
6
Two disjoint convex closed sets that cannot be separated by a closed hyperplane?
Jan 16 '17 at 7:54
6
Boundedness of Continuous Bilinear Operators
Oct 14 '10 at 6:09
5
Maximal ideals of $\mathbb{Z}[\sqrt{-5}]$
Dec 13 '16 at 12:16
5
Extension and restriction of scalars and their relation to the identity functor
Mar 19 '14 at 19:26
5
What are the subfields of $\mathbb{Q}(\sqrt{3},\sqrt[7]{5})$?
Mar 18 '14 at 14:57
5
A simple module versus a simple ring
Aug 8 '11 at 15:16
5
Representation of a reflection about a plane in $\mathbb{R}^3$
Feb 11 '11 at 8:12
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