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1d

answered  Number of double cosets of a Young subgroup 
1d

awarded  Explainer 
Sep 25 
comment 
Why is there a connection between enumerative geometry and nonlinear waves?
Some other references are the five arXiv papers by Yuji Kodama and Lauren Williams. 
Sep 17 
answered  Pictures of the von Neumann polytope 
Sep 13 
comment 
Flag complexes that are shellable but not vertex decomposable
Proposition 6.8(i) of arxiv.org/pdf/1303.2070.pdf gives an example of a nonshellable triangulation of a 3ball whose barycentric subdivision is vertexdecomposable. This suggests that there might be a nonshellable triangulation of a 3ball whose barycentric subdivision is shellable but not vertexdecomposable. 
Sep 9 
awarded  co.combinatorics 
Sep 9 
answered  Counting cyclic binary sequences of length $n$ where ones appear in blocks of length at least $k$ 
Sep 9 
revised 
Counting cyclic binary sequences of length $n$ where ones appear in blocks of length at least $k$
corrected spelling in title 
Aug 16 
comment 
Set of distinct real numbers such that all combination of sums are distinct
In fact, if we want the condition to hold for all $p$, then linear independence over $\mathbb{Q}$ is necessary and sufficient. 
Aug 15 
awarded  Famous Question 
Aug 11 
awarded  Nice Answer 
Jul 19 
answered  General criterion to find a Zbasis in a fixed generating subset 
Jul 17 
comment 
Combinatorial interpretation of composition of power series?
For the general theory of the formal power series identity $f(f(t))=t$, see Enumerative Combinatorics, vol. 1, Exercise 1.168. 
Jul 14 
comment 
Counting representations of $k[x,y]$ when $k$ is finite
Is it just a coincidence that $r_n(1)$ is the middle coefficient of $(1+x+x^2)^n$ for $1\leq n\leq 4$? 
Jul 14 
comment 
counting the number of ordered pairs in a permutohedron
People have looked at this kind of question, but I believe your particular question is open. See for instance Exercise 3.185 of *Enumerative Combinatorics", vol. 1. Your question is closely related to mathoverflow.net/questions/173042, since the number of pairs $\tau\leq\sigma$ is equal to $\sum_\sigma \#[\mathrm{id},\sigma]$. 
Jul 14 
awarded  Great Answer 
Jul 13 
awarded  Necromancer 
Jul 12 
answered  What is natural about the wellknown bijection between conjugacy classes and irreps of a symmetric group? 
Jul 11 
answered  Sums of subsets of $\mathbb{Z}/n\mathbb{Z}$ 
Jul 6 
comment 
Ordinary Generating Function for Bell Numbers
As an aside, let $G(t)=\sum_{n\geq 0}B_nt^n$. Then the solution to Exercise 1.111 of Enumerative Combinatorics, vol. 1, shows that $(1t)G(t(1t))$ is the ordinary generating function for the number of partitions of $1,2,\dots,n$ such that no block is an interval $a,a+1,\dots,b$ (including the case $a=b$). 