arXiv subjects

Mikael Hansson

Publications and source records attributed to Mikael Hansson.

5 recordsLinked to original sources

arXiv2023

Fixed elements of pircon automorphisms

We prove that the subposet induced by the fixed elements of any automorphism of a pircon is also a pircon. By a result of Abdallah, Hansson, and Hultman, the order complex of any open interval in a pircon is a PL ball or a PL sphere. We apply our main results to symmetric groups of the form $S_{2n}$. A consequence is that the fixed point free signed involutions form a pircon under the dual of the Bruhat order on the hyperoctahedral group. Finally, we prove that this poset is, in fact, EL-shellable, which is a type $B$ analogue of a result of Can, Cherniavsky, and Twelbeck.

math.CO↗

arXiv2017

Topology of posets with special partial matchings

Special partial matchings (SPMs) are a generalisation of Brenti's special matchings. Let a \emph{pircon} be a poset in which every non-trivial principal order ideal is finite and admits an SPM. Thus pircons generalise Marietti's zircons. We prove that every open interval in a pircon is a PL ball or a PL sphere. It is then demonstrated that Bruhat orders on certain twisted identities and quasiparabolic $W$-sets constitute pircons. Together, these results extend a result of Can, Cherniavsky, and Twelbeck, prove a conjecture of Hultman, and confirm a claim of Rains and Vazirani.

math.CO↗

arXiv2017

A word property for twisted involutions in Coxeter groups

Given an involutive automorphism $θ$ of a Coxeter system $(W,S)$, let $\mathfrak{I}(θ) \subseteq W$ denote the set of twisted involutions. We provide a minimal set of moves that can be added to the braid moves, in order to connect all reduced $\underline{S}$-expressions (also known as admissible sequences, reduced $I_θ$-expressions, or involution words) for any given $w \in \mathfrak{I}(θ)$. This can be viewed as an analogue of the well-known word property for Coxeter groups. It improves upon a result of Hamaker, Marberg, and Pawlowski, and generalises similar statements valid in certain types due to Hu, Zhang, Wu, and Marberg.

math.CO↗

arXiv2016

Generalised Ramsey numbers for two sets of cycles

We determine several generalised Ramsey numbers for two sets $Γ_1$ and $Γ_2$ of cycles, in particular, all generalised Ramsey numbers $R(Γ_1,Γ_2)$ such that $Γ_1$ or $Γ_2$ contains a cycle of length at most $6$, or the shortest cycle in each set is even. This generalises previous results of Erdős, Faudree, Rosta, Rousseau, and Schelp from the 1970s. Notably, including both $C_3$ and $C_4$ in one of the sets, makes very little difference from including only $C_4$. Furthermore, we give a conjecture for the general case. We also describe many $(Γ_1,Γ_2)$-avoiding graphs, including a complete characterisation of most $(Γ_1,Γ_2)$-critical graphs, i.e., $(Γ_1,Γ_2)$-avoiding graphs on $R(Γ_1,Γ_2)-1$ vertices, such that $Γ_1$ or $Γ_2$ contains a cycle of length at most $5$. For length $4$, this is an easy extension of a recent result of Wu, Sun, and Radziszowski, in which $|Γ_1|=|Γ_2|=1$. For lengths $3$ and $5$, our results are new even in this special case. Keywords: generalised Ramsey number, critical graph, cycle, set of cycles

math.CO↗

arXiv2015

The Bruhat order on conjugation-invariant sets of involutions in the symmetric group

Let $I_n$ be the set of involutions in the symmetric group $S_n$, and for $A \subseteq \{0,1,\ldots,n\}$, let \[ F_n^A=\{σ\in I_n \mid \text{$σ$ has $a$ fixed points for some $a \in A$}\}. \] We give a complete characterisation of the sets $A$ for which $F_n^A$, with the order induced by the Bruhat order on $S_n$, is a graded poset. In particular, we prove that $F_n^{\{1\}}$ (i.e., the set of involutions with exactly one fixed point) is graded, which settles a conjecture of Hultman in the affirmative. When $F_n^A$ is graded, we give its rank function. We also give a short new proof of the EL-shellability of $F_n^{\{0\}}$ (i.e., the set of fixed point-free involutions), which was recently proved by Can, Cherniavsky, and Twelbeck. Keywords: Bruhat order, symmetric group, involution, conjugacy class, graded poset, EL-shellability

math.CO↗