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Enumerative combinatorics, graph theory, order theory, posets, matroids, designs and other discrete structures. It also includes algebraic, analytic and probabilistic combinatorics.

7 votes

Partial order relation on subsets

This is the Bruhat order on $S_n / (S_k \times S_{n-k})$, which models the inclusion relations of Schubert varieties on the Grassmannian $Gr(k,n)$. The elements of $S_n / (S_k \times S_{n-k})$ are ge …
Michael Joyce's user avatar
3 votes
Accepted

Trying to understand the proof of Laurent phenomenon of cluster algebras

You need to use the fact that $x'_j = M x_q + M'$, where $M$ and $M'$ are Laurent monomials in the remaining variables. Their observation is that if $x'_j$ factored, it would be as $x'_j = (P_1 x_q + …
Michael Joyce's user avatar
4 votes

Continuous notions with compelling discrete analogues

Discrete difference equations generalize differential equations. In a similar spirit, divided difference operators generalize partial differentiation operators. Though such operators go back to Newt …
21 votes
1 answer
2k views

Why are the power symmetric functions sums of hook Schur functions only?

One interesting fact in symmetric function theory is that the power symmetric function $p_n$ can be written as an alternating sum of hook Schur functions $s_{\lambda}$: $$ p_n = \sum_{k+\ell = n} (-1) …
Michael Joyce's user avatar
3 votes

Is there a standard name for this poset

This is a manifestation of the Bruhat order on the Grassmannian $Gr(k,n)$ of $k$-planes in an $n$-dimensional vector space. In terms of partitions (Young diagrams): Given a set $X$ as above, let $\la …
Michael Joyce's user avatar
2 votes

Covering relations in $K\backslash G/B$

While the paper of Richardson-Springer does study the weak order, it also has useful results on the usual (strong) Bruhat order. In particular, Theorem 7.11 says that Bruhat order is characterized as …
Michael Joyce's user avatar
4 votes

RSK and crystal operators

There is a detailed analysis in Chapters 7 and 8 of Bump and Schilling's Crystal Bases. They work through the connection between RSK and crystals in careful detail, though I don't recall how much deta …
Michael Joyce's user avatar
5 votes

Good combinatorics textbooks for teaching undergraduates?

Neither of these suggestions seem to exactly fit the level the OP was aiming for, but I add them for others who come across this thread with a different group of students in mind: (1) For a gentle, p …
10 votes
Accepted

Is there a geometric interpretation of skew Schur functions?

This is discussed in Stanley's paper Some combinatorial aspects of the Schubert calculus. Corollary 3.7 says that under the natural isomorphism given by the Borel presentation of $H^*(G/P)$ which send …
Michael Joyce's user avatar