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Norbert
  • Member for 12 years, 6 months
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122 votes
1 answer
4k views

Continuous projections on $\ell_1$ with norm $>1$

31 votes
1 answer
7k views

Is it true that $\mathbb{R}$ and $\mathbb{R}^2$ are isomorphic as abelian groups?

29 votes
2 answers
6k views

Direct proof that nilpotent matrix has zero trace

24 votes
5 answers
2k views

Computing $\lim\limits_{n\to+\infty}n\int_{0}^{\pi/2}xf(x)\cos ^n xdx$

22 votes
1 answer
4k views

Uniform convergence of series $\sum\limits_{n=2}^\infty\frac{\sin n x}{n\log n}$

19 votes
4 answers
3k views

Solving functional equation $f(x)^2+f(y)^2=f(x+y)(f(f(x))+f(y))$

18 votes
6 answers
1k views

How to compute $\sum\limits_{k=0}^n (-1)^k{2n-k\choose k}$?

17 votes
1 answer
536 views

Minmax problem for polygons

17 votes
4 answers
800 views

Prove that $(a-d)^2+(b-c)^2\geq 1.6$ if $a^2+4b^2=4$ and $cd=4$

13 votes
2 answers
3k views

A counterexample to theorem about orthogonal projection

13 votes
1 answer
2k views

Matrices with entries in $C^*$-algebra

11 votes
1 answer
1k views

Isometry between $L_\infty$ and $\ell_\infty$

11 votes
1 answer
496 views

Proving a certain determinant $\left|\det A\right|$ is complete square

9 votes
1 answer
950 views

Uncountable product in the category of metric spaces.

9 votes
1 answer
279 views

How to compute $\int\limits_{\mathbb{R}^3}\det D^2 u(\mathbf{r}) d\mathbf{r}$?

8 votes
2 answers
140 views

Question about swindler-writer

8 votes
2 answers
2k views

Convergence of test-functions is not induced by any metric.

7 votes
2 answers
2k views

Proof of Pitt's theorem

6 votes
1 answer
287 views

When only eventually constant sequences are convergent?

6 votes
2 answers
300 views

Examples of almost compact spaces.

6 votes
0 answers
180 views

Reference request for the fact

5 votes
1 answer
182 views

Is $\mathcal{B}(H)$ complemented in $\ell_\infty(I, H)$

5 votes
1 answer
278 views

Extension of Goldstine theorem

5 votes
1 answer
159 views

Characterization of pretty compact spaces

5 votes
1 answer
107 views

Measure without sets of measure zero

5 votes
0 answers
207 views

Measurability of the max function.

5 votes
2 answers
958 views

When multiplication operator is an bounded below/ open mapping/isometry/quotient map?

5 votes
1 answer
206 views

Properties of certain type of measures

5 votes
1 answer
199 views

How to prove $\mathrm{Im}(\mathrm{Ext}_R^1(g,A'))=\mathrm{Ker}(\mathrm{Ext}_R^1(f,A'))$

5 votes
2 answers
935 views

Counting matrices over $\mathbb{Z}/2\mathbb{Z}$ with conditions on rows and columns