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40m

comment 
reference for groupoid cohomology
This is essentially just fancy language for the statement that the cohomology of a discrete group $G$ can be computed as the cohomology of the space $K(G,1)$. 
4h

comment 
Can the projective line be provided with a ring structure?
@WolfgangTintemann: Yes, it is obtained by conjugating addition on $\mathbb{R}/\pi\mathbb{Z}$ by the homeomorphism $\tan:\mathbb{R}/\pi\mathbb{Z}\to\mathbb{P}^1_{\mathbb{R}}$. 
19h

revised 
When did people know that all real polynomials of degree greater than 2 are reducible?
edited tags 
1d

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Reference for (co)limitpreserving functor $X\mapsto R^X$
I wonder what can be said in general about (say, presentable) categories $\mathcal{C}$ such that the functor $\text{set}^{op}\to\mathcal{C}$ sending $X$ to the power $I^X$ preserves finite colimits, where $I$ is the initial object. Everything I said here applies to any such category; in particular, the functor $\mathcal{C}\to\text{Set}$ corepresented by the object $I^2$ can be lifted to a right adjoint $G:\mathcal{C}\to\text{Bool}$ that in some sense provides $\mathcal{C}$ with robust notion of "idempotents". 
1d

revised 
Reference for (co)limitpreserving functor $X\mapsto R^X$
added 710 characters in body 
1d

answered  Reference for (co)limitpreserving functor $X\mapsto R^X$ 
Jul 30 
answered  Adjointable Abelian Monoidal Functor 
Jul 29 
reviewed  No Action Needed What am I missing in this highly oscillatory integral? 
Jul 26 
comment 
Is there a generalization of homotopy groups to fractional dimensions
If there were a "$1/2$sphere" $S^{1/2}$, you would probably expect it to be a pointed space for which $S^{1/2}\wedge S^{1/2}$ is weak equivalent to $S^1$. It is easy to show (using homology, for instance) that no such space exists. This doesn't prove there is no sensible notion of $\pi_{1/2}$, but it is some evidence against it. 
Jul 26 
comment 
Bohr compactification and “discretization”
Yes, that's right. 
Jul 26 
comment 
On the global structure of the GromovHausdorff metric space
For any $r>0$, there is an autohomeomorphism of $\mathcal{GH}$ that takes a compact metric space and scales its metric by $r$. 
Jul 25 
answered  Bohr compactification and “discretization” 
Jul 24 
comment 
Which combinations of normality, separability, and paracompactness do complex manifolds possess?
How do you define holomorphic charts on your example? 
Jul 24 
comment 
Which combinations of normality, separability, and paracompactness do complex manifolds possess?
A complex manifold must have a set of charts with holomorphic transition functions, not just be locally homeomorphic to $\mathbb{C}$. 
Jul 24 
accepted  LöwenheimSkolem for manysorted theories 
Jul 23 
reviewed  Approve LöwenheimSkolem for manysorted theories 
Jul 23 
awarded  Good Answer 
Jul 23 
revised 
LöwenheimSkolem for manysorted theories
deleted 326 characters in body 
Jul 23 
comment 
LöwenheimSkolem for manysorted theories
@Goldstern: Ah, very nice! So the problem is definitely more complicated than I thought. Let me edit the question a bit to reflect this. 
Jul 23 
comment 
(A kind of) Irreducibiliy of regular open convex sets in the Cartesian space
Given any point $y$ in $V$, if you draw a line segment from $y$ to $x$, and then extend the line segment a little bit past $x$, you will enter the set $W\cdot U$. Thus $x$ lies on the line segment joining $y$ to some point in $W\cdot U$. Since $W\cdot U\leq C$ and $C$ is convex, $y\in C$ would imply $x\in C$. 