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9h

comment 
Counterexample on completely distributive lattices
In case it helps, I have written out a proof that completely distributive Boolean algebras are atomic, in the nLab here: ncatlab.org/nlab/show/… 
17h

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Fixed point theorems
@WłodzimierzHolsztyński The "question" asks for interesting examples of fixed point theorems, in the interest of compiling a big list. 
21h

revised 
Counterexample on completely distributive lattices
a few more words of explanation 
21h

answered  Counterexample on completely distributive lattices 
1d

awarded  Nice Answer 
1d

revised 
A question about summation formula involving binomial coefficient
corrected a typo; slight rewording for more clarity 
1d

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Why are the integers with the cofinite topology not pathconnected?
Well, you need something: any function from an interval to an indiscrete space is continuous, so indiscrete spaces are pathconnected. They cannot be arcconnected if they are countable. That a pathconnected Hausdorff space is arcconnected is a pretty nontrivial theorem. 
2d

answered  A question about summation formula involving binomial coefficient 
2d

revised 
Is every frame monomorphism regular?
corrected a typo 
2d

revised 
Is every frame monomorphism regular?
corrected a typo 
2d

answered  Is every frame monomorphism regular? 
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answered  Extremal, but not regular monomorphism 
2d

revised 
inverted factorial and trailing zeros problem
tried fixing the link 
Apr 14 
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How to integrate complex numbers?
This site is for professional mathematicians and their PhD students to discuss their research. Mathematical questions outside of that scope may be asked at math.stackexchange. However, the present question might be closed there too as it hasn't been expressed with much care  it might be better to talk to your instructor or a teaching assistant. 
Apr 13 
awarded  Nice Answer 
Apr 13 
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How many subspaces are generated by three or more subspaces in a Hilbert space?
That's because, I think, the general case is extremely hard. Even the case $n = 4$ is apparently really hard. One can obtain infinitely many subspaces from 4 subspaces in general position; see sciencedirect.com/science/article/pii/S0022404907000217 and Rota's article "Ten mathematics problems I will never solve" (behind paywall: degruyter.com/view/j/dmvm.1998.6.issue2/dmvm19980215/…). 
Apr 13 
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Snakelike continua and universal images
@DavidWhite It's there by intention, as several recent discussions have borne out. Probably no one but the author understands why he typesets that way, but I think it would be wise not to pursue this here and now. The mathematics looks interesting. 
Apr 13 
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Answer to “why is matrix called matrix and what does it have to do with the movie?”
I have a feeling this will be considered offtopic. You might consider posting to History of Math and Science. But "matrix" comes from the Latin (ultimately deriving from mater, mother), and one should just look it up: Merriam online says "something within or from which something else originates, develops, or takes form". The movie meaning fits with that. The word as used in mathematics was introduced by Sylvester: en.wikipedia.org/wiki/Matrix_%28mathematics%29#History 
Apr 12 
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klinear abelian categories which are not categories of modules
I have a suspicion that something like chain complexes whose total space is finitedimensional would be a counterexample, but I don't have time to look into it at the moment. 
Apr 12 
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Can there be a nontrivial epimorphism (of rings) from a field?
@tj_ I expect what you mean is that $Tor$ commutes with filtered or directed colimits, not to be confused with direct limits = colimits. But your argument still goes through. And perhaps more to the point, I take it that you meant to offer a choicefree argument (I already knew modules over fields are flat assuming choice), and the argument seems valid in that respect as well. Thanks! 