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3h

comment 
Derivative in terms of finite differences
Link to Jordan's book: books.google.com/… 
5h

awarded  Necromancer 
7h

comment 
An amenable group containing a wreath product of itself
@HJRW Thanks for the information. 
16h

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An amenable group containing a wreath product of itself
I thought a wreath product involved products, not sums, e.g., $(\prod_{n \in \mathbb{Z}} G) \rtimes \mathbb{Z}$. I guess you mean "restricted wreath product": en.wikipedia.org/wiki/Wreath_product#Definition ? 
16h

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Two conjectures in number theory
Please don't ask us to visit your blog: MathOverflow is not to be used as an advertising space. You can put your blog info in your user profile if you like. 
17h

comment 
Two conjectures in number theory
Stanley is right. You might also read the information in the help section such as "how to ask", and look at highly rated questions with the numbertheory tag, to get an idea of how questions and answers generally work here. Finally, you should exercise more care in formulating your conjectures, to rule out cases you obviously don't want (e.g. all the $a_i$ are zero). 
1d

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Can the projective line be provided with a ring structure?
If $\phi: X \to Y$ is a bijection and $X$ carries a group structure (denoted by $+_X$), then $Y$ carries a group structure by "conjugation": $y +_Y y' = \phi(\phi^{1}(y) +_X \phi^{1}(y'))$. This I'm sure is what Eric meant. Another term for this (which generalizes to any sort of structure you like) is "transport of structure". It's a general thing all mathematicians pick up on at some point. 
1d

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Importance of separability vs. secondcountability
@PaulTaylor Please let us know (either here or at the nForum: nforum.ncatlab.org) if you have spotted an error in the nLab article. 
1d

answered  About a closed strucure on profunctors 
Jul 28 
awarded  Nice Answer 
Jul 14 
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Euler characteristic of a curve
@AlexDegtyarev There are plenty of research mathematicians who may stumble upon a phenomenon in an area where they are not expert. MathOverflow is tailormade for such mathematicians to get expert responses, often even more illuminating than what they might find through Google searches. (I often feel we've drifted too far away from the original vision of MO, which was really exactly in view of such situations.) 
Jul 14 
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Euler characteristic of a curve
I think it would be a shame to close this question. Something whose explanation comes from Hodge theory and HirzebruchRiemannRoch doesn't "belong" at M.SE. 
Jul 12 
awarded  Good Question 
Jul 12 
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Good references for Rigged Hilbert spaces?
Could you comment then on why, for you, GelfandVilenkin falls short? As it stands, this answer is not very useful to me, but if you could comment on this it might become much more useful. 
Jul 12 
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Do modelsandhomomorphisms always form an accessible category?
A belated welcome to MO, Professor Rosický. Looking forward to reading more from you. 
Jul 12 
revised 
Do modelsandhomomorphisms always form an accessible category?
fixed LaTeX 
Jul 10 
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Does the fat realization of simplicial spaces commute with finite limits up to homotopy?
I don't quite understand the vote to close. 
Jul 10 
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Two limit cycles which lie on the same leaf
Meta post: meta.mathoverflow.net/questions/2349/downvotewithoutcomment 
Jul 8 
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Does “$\forall Z(C(X,Z) \cong C(Y,Z))$” imply $X\cong Y$?
To add to what Chris is saying: if $1$ is the onepoint space, then for any space $A$ the homset $\hom_{Top}(1, A)$ is the underlying set of $A$. In the case of $A = C(X, Z)$, no matter what topology we put on $C(X, Z)$ and no matter what the topological assumptions are on $X$, we get $\hom(1, C(X, Z)) \cong \hom(X, Z)$, since the points of $C(X, Z)$ are by definition (parametrized by) maps $X \to Z$. Applying $\hom(1, )$ to a natural isomorphism $C(X, Z) \cong C(Y, Z)$, we then get a natural isomorphism $\hom(X, Z) \cong \hom(Y, Z)$, at which point Yoneda applies. 
Jul 7 
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If a result is apparently provable with AC, is actually independent of ZF?
To say that it was first obtained by Lindenbaum and Tarski in 1926 is to skip over the actual history of the situation. Certainly it was not published in 1926, and Tarski many years later said he couldn't remember how Lindenbaum's argument for a key lemma went, but found a proof for it himself. The (Conway)Doyle paper describes all this better than I can in a small comment box (they, or he [Doyle], surmise that they hit upon the argument Lindenbaum must have used, but I'd treat that as just a guess). 