## Tagged Questions

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### Lawvere’s fixed point theorem and the Recursion Theorem

Building off of Qiaochu's comment on my answer to a previous mathoverflow question (http://mathoverflow.net/questions/130883/is-there-any-proof-that-you-feel-you-do-not-understand/ …
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### An integral with Gamma functions (Part 2)

I was wondering if there is a generalization of the integral discussed here to a case like, \int \frac{d^dq}{q^{\nu_1}\vert \vec{q} \pm \vec{k}_1\vert ^{\nu_2}\ver …
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### How do I find abelian subcategories of periodic triangulated categories?

If $T$ is a triangulated category, then the formalism of $t$-structures gives a way to find abelian subcategories inside. You're supposed to find two strictly full subcategories, …
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### Question about the nullstellensatz for projective schemes

Yeasterday I asked a question on math stackexchange which simplifies to the following: Assume that $G$ is a graded ring and $A \subseteq G$ is a homogeneous radical ideal. …
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### Connected groupoids and action groupoids

It is written in Wikipedia http://en.wikipedia.org/wiki/Groupoid, that any connected groupoid $A\rightrightarrows X$ is isomorphic to an action groupoid $G\ltimes X$ coming from a …
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### A duality on partial permutations

A partial permutation matrix $\pi$ is one with at most one 1 in any row and column (the rest 0s). Given one, we can cross out to the East and South (but not Southeast) of each 1. S …
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### How many orders of infinity are there?

Define a growth function to be a monotone increasing function $F: {\bf N} \to {\bf N}$, thus for instance $n \mapsto n^2$, $n \mapsto 2^n$, $n \mapsto 2^{2^n}$ are examples of grow …
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### Why is Set, and not Rel, so ubiquitous in mathematics?

The concept of relation in the history of mathematics, either consciously or not, has always been important: think of order relations or equivalence relations. Why was there the n …
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### Octonions and the Fano plane.

Does the Fano plane mnemonic for octonion multiplication have any deeper meaning? http://upload.wikimedia.org/wikipedia/commons/2/2d/FanoPlane.svg The symmetry group of the Fano …
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The action of a group $G$ on a topological space $X$ can be viewed as a functor $F: G \to \mathcal{Top}$ with $F(*)=X$. (Here I'm viewing a group as a category with one object, $* … 1answer 119 views ### Set Theory exercise. I find myself unable to solve question 24.1 of T. Jech's Set Theory: If$\beta<\omega_1$and if$2^{\aleph_{\alpha}}\leq\aleph_{\alpha+\beta}$for a stationary set of$\ …
Let $f \in C^{\gamma}_c(\mathbb{R})$. Let $K:\mathbb{R}^n \backslash {\vec{0}} \rightarrow \mathbb{R}^n$ be a singular integral kernel with the following properties: 1) K smooth …