bio  website  thales.doa.fmph.uniba.sk/… 

location  Bratislava, Slovakia  
age  36  
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1d

comment 
A book for problems in Functional Analysis
Also Fabian, Habala, Hájek, Montesinos, Zizler: Banach Space Theory, The Basis for Linear and Nonlinear Analysis has a lot of exercises. 
1d

revised 
A book for problems in Functional Analysis
minor typo 
May 17 
comment 
On the independence of lower and upper asymptotic and Banach densities
Changing a set $A$ to $B=\{\lfloor ca \rfloor; cA\}$ for $c\in(0,1)$ changes all four densities with factor $c$. So it is sufficient to prove the case $\delta=1$. (Which is not much of a simplification, but perhaps it helps at least a bit.) 
May 16 
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On the independence of lower and upper asymptotic and Banach densities
I just want to confirm that I cannot find a reference you ask for and I have probably seen result for some other kind of densities. (Which means the my answer back then was partially misleading. Although, this was probably not the most important part of that answer.) 
May 16 
revised 
Density of a set of natural numbers whose differences are not bounded.
added 243 characters in body 
May 1 
revised 
Which popular games are the most mathematical?
old link to an image was not working; I replaced it by imgur link converted from https://upload.wikimedia.org/wikipedia/commons/4/4b/Sokoban_ani.gif 
Feb 19 
revised 
Uniqueness of minimal completions of a partially ordered set
typos: MacNielle > MacNeille; added links 
Feb 19 
suggested  approved edit on Uniqueness of minimal completions of a partially ordered set 
Jan 15 
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name for “solid” subset of a partially ordered set?
It is probably not worth bumping 5 years old post with such a trivial edit, but for the sake of better readability, the condition is: If $a,c\in S$ and $b\in P$ and $a\le b\le c$ then $b\in S$. 
Jan 10 
revised 
Best online mathematics videos?
added (onlineresources) tag; this tag was discussed here: http://chat.stackexchange.com/rooms/10243/conversation/onlineresourcestag 
Dec 28 
comment 
Subset of the plane that intersects every line exactly twice
There is also freely available paper Erdös, P., and Bagemihl, F.. "Intersections of prescribed power, type, or measure." Fundamenta Mathematicae, eudml.org/doc/213347, matwbn.icm.edu.pl/ksiazki/fm/fm41/fm4119.pdf 
Dec 28 
revised 
Subset of the plane that intersects every line exactly twice
minor typo; added jstor link 
Dec 28 
suggested  approved edit on Subset of the plane that intersects every line exactly twice 
Dec 12 
revised 
How prove this polynomial inequality from a book
minor typos 
Dec 12 
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How prove this polynomial inequality from a book
Here is Google Books link to the reference given by Lucia. I would be quite curious to know the name of the book where you saw this problem. 
Dec 12 
suggested  approved edit on How prove this polynomial inequality from a book 
Dec 3 
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Hausdorff spaces with trivial automorphism group
There is also a series of papers by Kannan and Rajagopalan on the topic of rigid spaces. You can find there some further constructions and references. They also study strongly rigid spaces, which you mention in another question. 
Nov 18 
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StoneCech compatification and ultrafilter
You can also have a look at these notes of mine which include proofs of some facts about ultrafiter construction of $\beta\mathbb N$. I would say the proofs there are quite detailed. 
Nov 18 
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StoneCech compatification and ultrafilter
If you can be more specific, perhaps you could make a suitable question for math.SE from this. Asking not so specific question seems to be more suitable for chat. Over there at math.SE we have General Topology chatroom which is rather inactive these days. However, you need at least 20 reputation points to talk in chat. I am not sure whether you need 20 points on math.SE or 20 points on any site would suffice. 
Nov 2 
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“a shape that … lies halfway between a square and a circle”
Since my suggested edit was rejected, I'll just point out in a comment that it is preferred to embed images using the editor/imgur instead of using external links to prevent link rot (as discussed on meta). 