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Seirios's user avatar
Seirios's user avatar
Seirios
  • Member for 11 years, 11 months
  • Last seen more than a month ago
131 votes
Accepted

Is the inverse of a symmetric matrix also symmetric?

73 votes
Accepted

$1=2$ | Continued fraction fallacy

47 votes
Accepted

Is it always true that $\det(A^2+B^2)\geq0$?

41 votes

"It looks straightforward, but actually it isn't"

40 votes

A Banach space is reflexive if and only if its dual is reflexive

39 votes

Your favourite application of the Baire Category Theorem

29 votes
Accepted

Additivity + Measurability $\implies$ Continuity

27 votes

Does $G\times K\cong H\times K$ imply $G\cong H$?

27 votes

How to prove that if a normed space has Schauder basis, then it is separable? What about the converse?

24 votes

Is the closure of a convex set again a convex set?

22 votes

A question on countability of isolated points of a subset of R

21 votes

Nice examples of groups which are not obviously groups

19 votes

How to prove $\det \left(e^A\right) = e^{\operatorname{tr}(A)}$?

18 votes

Why must a finite covering map be closed?

18 votes
Accepted

Weak topology on an infinite-dimensional normed vector space is not metrizable

18 votes
Accepted

If $f'(x) = 0$ for all $x \in \mathbb{Q}$, is $f$ constant?

17 votes

Why the group $\langle x,y\mid x^2=y^2\rangle $ is not free?

16 votes
Accepted

Holomorphic function with zero derivative is constant on an open connected set

16 votes
Accepted

$ \sum_{k=1}^{\infty} \ln{\left(1 + \frac{1}{4 k^2}\right)}$ Computing this sum

15 votes

Give an example of a noncyclic Abelian group all of whose proper subgroups are cyclic.

15 votes
Accepted

Find an abelian infinite group such that every proper subgroup is finite

15 votes
Accepted

How to show the standard $n$-simplex is homeomorphic to the $n$-ball

14 votes

Prove that $\det(A)\neq 0$.

14 votes
Accepted

If $A^2+2A+I_n=O_n$ then $A$ is invertible

14 votes
Accepted

Find $f:\mathbb R\to\mathbb R$ such that $f(x)=f'(x)$.

13 votes
Accepted

Is the Riemann integral of a strictly positive function positive?

13 votes

Trying to define $\mathbb{R}^{0.5}$ topologically

13 votes

What are some motivating examples of exotic metrizable spaces

12 votes

Difference between complete and closed set

12 votes

$G$ is a finite group and $G_1$,$G_2$ are subgroup that $G_1 \cap G_2=\{1\}$, then $|G_1|.|G_2| \big| |G|$?

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