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Mar
30
comment Counter-example to exponential law for locally compact [non-Hausdorff] spaces
...the idea of taking finite $X$ and $Y$ is quite tempting — because finite topological spaces are locally-compact (and I don't know many example of non-Hausdorff locally-compact spaces). I've tried to play with some examples — but haven't succeeded.
Mar
30
comment Counter-example to exponential law for locally compact [non-Hausdorff] spaces
@johndoe Well, if either $X$, $Y$ or $Z$ is discrete the statement seems to be more or less obviously true. But slightly more generally...
Mar
30
comment Counter-example to exponential law for locally compact [non-Hausdorff] spaces
Related (but not answering the question): @ n-Category Cafe
Mar
30
asked Counter-example to exponential law for locally compact [non-Hausdorff] spaces
Mar
21
comment Combinatorial definition of Hall–Littlewood polynomials (sum over SSYT?)
@IgorMakhlin (И про t-версию Бриона и т.п. мы бы с М.Б. с интересом послушали в какой-то момент.)
Mar
21
comment Combinatorial definition of Hall–Littlewood polynomials (sum over SSYT?)
@IgorMakhlin О, привет. Well, yes and no: there is an explicit description of the $t$-weight $\psi$ in Macdonald's book — but it's complicated and not terribly satisfying. So if you have a better answer, please explain it (here or iRL).
Mar
20
revised Algebraic tricks like componendo dividendo
edited tags
Mar
12
comment Geometric interpretation for sum of fourth powers
Well, yes, $1^k+2^k+...+n^k$ is the value of an Ehrhart polynomial for the 'hybercubic pyramid'. But does this help to compute the sum?
Jan
30
comment Is reduced homology a full functor on connected spaces?
The question is about the category of connected spaces.
Jan
30
comment Is reduced homology a full functor on connected spaces?
Uh? $\tilde H(pt)=0$.
Jan
23
reviewed Close In a group of order 21, every normal subgroup is cyclic
Jan
23
reviewed Close Separable spaces and functions that separate points
Jan
22
comment Curious Binomial Coefficient Identity
it's a (yet another) form of Vandermonde's identity
Jan
20
reviewed Close How to understand the regular cardinal?
Jan
20
reviewed Approve mantel theorem bipartite graphs, two triangles share an edge
Jan
19
reviewed Close I need help to evaluate this definite integral.
Jan
19
reviewed Close Solve 10 base logarithms
Jan
19
reviewed Close Forming equations for exponential growth/decay questions
Jan
19
comment Binomial Identity $\sum\binom{2n+1}{2k+1}\binom{m+k}{2n} = \binom{2m}{2n}$
see also math.stackexchange.com/q/1107465 for a bijective proof of an equivalent identity
Jan
19
comment Showing ${n + a - 1 \choose a - 1} = \sum_{k = 0}^{\left\lfloor n/2 \right\rfloor} {a \choose n-2k}{k+a-1 \choose a-1}$
come to think of it, substituting $a\to 2n+1$, $n\to 2m-2n$ (and $k\to m-2n+k$) one can see that these two identities are equivalent