bio  website  math.berkeley.edu/~qchu 

location  Berkeley, CA  
age  24  
visits  member for  5 years, 2 months 
seen  42 mins ago  
stats  profile views  55,953 
I am a thirdyear graduate student at UC Berkeley. I'm interested in the interplay between homotopy theory, quantum field theory, manifold topology, and higher category theory.
6h

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What is a totient?
@Joonas: so "totient function" means a function counting the number of times something happens? 
6h

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What is a totient?
I think the question is why such a general definition deserves the name "totient." Is it just by analogy with the Euler totient function or is there some general nonmathematical meaning of the word "totient" that is being invoked here? 
1d

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Cantor's theorem for presheaves?
@Michal: this is obvious if you require that $C$ is small. I think Todd wants to take $C$ to be $U$small relative to a universe $U$ and then I guess things are okay if $\text{Set}$ is also the $U$small version? I'm not very familiar with exactly what operations are and aren't allowed when working with universes, and in particular I don't know anything about forming $U$small functor categories. 
1d

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Cantor's theorem for presheaves?
@Theo: I think there are at least two problems with that argument, one being that at best you'll prove the statement with "essentially surjective" replaced by "surjective," but the other being that that to get a cartesian closed category of categories we need to restrict our attention to small categories, but then we disallow $\text{Set}$ itself... 
1d

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What's so special about $1$categories?
@Fernando: my understanding is that the standard convention is that "bicategory" means weak and "$2$category" means strict, but this convention doesn't generalize past "tricategory" and so forth to an arbitrary category number, and it seems perverse to have "$n$category" mean strict by default. 
2d

revised 
What's so special about $1$categories?
deleted 15 characters in body 
2d

answered  What's so special about $1$categories? 
Dec 20 
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Purely noncommutative algebraMorita equivalence
@truebaran: yes, that's what I meant by "the center is Morita invariant." 
Dec 19 
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What is the universal property of quotienting a normaliser of the subgroup?
I should mention that when I say "in more general contexts" I have in mind the following general question: you have a functor $F : C \to D$ and an object $c \in C$ to which you apply the functor to get an object $F(c)$. What extra structure does $F(c)$ have that reflects that it was obtained in this way? Said another way, what kind of "descent data" can we attach to $F(c)$? The starting observation is that at the very least we can attach an action of the natural automorphism group or endomorphism monoid of $F$ itself, but we can go farther than this in various ways. 
Dec 19 
revised 
What is the universal property of quotienting a normaliser of the subgroup?
deleted 5 characters in body 
Dec 19 
awarded  Popular Question 
Dec 19 
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Reconstruction Conjecture holds for Directed Acyclic Graphs?
Is there an additional meaning to the use of the word "deck" here that isn't carried by "set" (or maybe "multiset")? 
Dec 19 
revised 
What is the universal property of quotienting a normaliser of the subgroup?
added 117 characters in body 
Dec 19 
answered  What is the universal property of quotienting a normaliser of the subgroup? 
Dec 18 
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Lefschetz fixed notation
I don't see the problem. One of them has a subscript and the other doesn't. 
Dec 15 
awarded  Good Question 
Dec 15 
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Purely noncommutative algebraMorita equivalence
I should mention that nothing preceding the example is needed to justify the example, only to suggest where to look for it: it's clear that $\mathbb{H}$, and more generally any noncommutative division ring, is not Morita equivalent to a commutative algebra because the endomorphism algebra of every $\mathbb{H}$module is noncommutative. 
Dec 15 
awarded  Nice Answer 
Dec 15 
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A Categoryish Structure with Morphism Domains containing Multiple Objects?
en.wikipedia.org/wiki/Multicategory 
Dec 14 
answered  Purely noncommutative algebraMorita equivalence 