arXiv subjects

S. P. Glasby

Publications and source records attributed to S. P. Glasby.

At least 19 recordsLinked to original sources

arXiv2026

Mathematics of the NYT daily word game Waffle

This note investigates the combinatorics of permutations underlying the NYT daily word game Waffle. It helps to solve Waffle games and helps to understand why some games are easy to solve while others are very hard. It shows that a perfect unscrambling must have precisely 11 orbits, with at least one of length 1, on the 21 Waffle squares. It also describes practical algorithms for solving Waffle games and creating new games with extreme properties.

math.HO↗

arXiv2026

The probability that two elements with large $1$-eigenspaces generate a classical group

With high probability, among $O(\log n)$ independent randomly selected elements from a finite $n$-dimensional classical group, some pair of elements power to a $2$-element generating set for a naturally embedded classical subgroup of dimension $O(\log n)$. The $2$-element generating set produced consists of certain elements with large $1$-eigenspaces, called stingray elements. Underpinning this result is a new theorem on the generation of a finite classical group by a pair of stingray elements. In particular we show that, for classical groups not containing ${\rm SL}_n(q)$, the probability of generation is at least $0.975$. The explicit probability bounds we obtain will be applied to justify complexity analyses for new constructive recognition algorithms for finite classical groups.

math.GR↗

arXiv2026

Hilbert's Theorem 90, periodicity, and roots of Artin-Schreier polynomials

Let $E/F$ be a cyclic field extension of degree $n$, and let $σ$ generate the group ${\rm Gal}(E/F)$. If ${\rm Tr}^E_F(y)=\sum_{i=0}^{n-1}σ^i y=0$, then the additive form of Hilbert's Theorem 90 asserts that $y=σx-x$ for some $x\in E$. When $E$ has characteristic $p>0$ we prove that $x$ gives rise to a periodic sequence $x_0,x_1,\dots$ which has period $pn_p$, where $n_p$ is the largest $p$-power that divides $n$. We also show, if $y$ lies in the finite field $\mathbb{F}_{p^n}$, then the roots of a reducible Artin-Schreier polynomial $t^p-t-y$ have the form $x+u$ where $u\in\mathbb{F}_p$ and $x=\sum_{i=0}^{n-1}\sum_{j=0}^{i-1}z^{p^j}y^{p^i}$ for some $z\in\mathbb{F}_{p^e}$ with $e=n_p$. Furthermore, the sequence $\left(\sum_{j=0}^{i-1}z^{p^j}\right)_{i\ge0}$ is periodic with period $pe$.

math.NT↗

arXiv2025

Bipartite $q$-Kneser graphs and two-generated irreducible linear groups

Let $V:=(\mathbb{F}_q)^d$ be a $d$-dimensional vector space over the field $\mathbb{F}_q$ of order $q$. Fix positive integers $e_1,e_2$ satisfying $e_1+e_2=d$. Motivated by analysing a fundamental algorithm in computational group theory for recognising classical groups, we consider a certain quantity $P(e_1,e_2)$ which arises in both graph theory and group representation theory: $P(e_1,e_2)$ is the proportion of $3$-walks in the `bipartite $q$-Kneser graph' $Γ_{e_1,e_2}$ that are closed $3$-arcs. We prove that, for a group $G$ satisfying ${\rm SL}_d(q)\leqslant G\leqslant{\rm GL}_d(q)$, the proportion of certain element-pairs in $G$ called `$(e_1,e_2)$-stingray duos' which generate an irreducible subgroup is also equal to $P(e_1,e_2)$. We give an exact formula for $P(e_1,e_2)$, and prove that $1-q^{-1}-q^{-2}< P(e_1,e_2)< 1-q^{-1}-q^{-2}+2q^{-3}-2q^{-5}$ for $2\leqslant e_2\leqslant e_1$ and $q\geqslant2$.These bounds have implications for the complexity analysis of the state-of-the-art algorithms to recognise classical groups, which we discuss in the final section.

math.GR↗

arXiv2024

Absolutely irreducible quasisimple linear groups containing elements of order a specified Zsigmondy prime

This paper is concerned with absolutely irreducible quasisimple subgroups $G$ of a finite general linear group $GL_d(\mathbb{F}_q)$ for which some element $g\in G$ of prime order $r$, in its action on the natural module $V=(\mathbb{F}_q)^d$, is irreducible on a subspace of the form $V(1-g)$ of dimension $d/2$. We classify $G,d,r$, the characteristic $p$ of the field $\mathbb{F}_q$, and we identify those examples where the element $g$ has a fixed point subspace of dimension $d/2$. Our proof relies on representation theory, in particular, the multiplicities of eigenvalues of $g$, and builds on earlier results of DiMuro.

math.RT↗

arXiv2024

Maximizing weighted sums of binomial coefficients using generalized continued fractions

Let $m,r\in\mathbb{Z}$ and $ω\in\mathbb{R}$ satisfy $0\leqslant r\leqslant m$ and $ω\geqslant1$. Our main result is a generalized continued fraction for an expression involving the partial binomial sum $s_m(r) = \sum_{i=0}^r\binom{m}{i}$. We apply this to create new upper and lower bounds for $s_m(r)$ and thus for $g_{ω,m}(r)=ω^{-r}s_m(r)$. We also bound an integer $r_0 \in \{0,1,\dots,m\}$ such that $g_{ω,m}(0)<\cdots \cdots>g_{ω,m}(m)$. For real $ω\geqslant\sqrt3$ we prove that $r_0\in\{\lfloor\frac{m+2}{ω+1}\rfloor,\lfloor\frac{m+2}{ω+1}\rfloor+1\}$, and also $r_0 =\lfloor\frac{m+2}{ω+1}\rfloor$ for $ω\in\{3,4,\dots\}$ or $ω=2$ and $3\nmid m$.

math.NT↗

arXiv2023

Derangements in wreath products of permutation groups

Given a finite group $G$ acting on a set $X$ let $δ_k(G,X)$ denote the proportion of elements in $G$ that have exactly $k$ fixed points in $X$. Let $\mathrm{S}_n$ denote the symmetric group acting on $[n]=\{1,2,\dots,n\}$. For $A\le\mathrm{S}_m$ and $B\le\mathrm{S}_n$, the permutational wreath product $A\wr B$ has two natural actions and we give formulas for both, $δ_k(A\wr B,[m]{\times}[n])$ and $δ_k(A\wr B,[m]^{[n]})$. We prove that for $k=0$ the values of these proportions are dense in the intervals $[δ_0(B,[n]),1]$ and $[δ_0(A,[m]),1]$. Among further result, we provide estimates for $δ_0(G,[m]^{[n]})$ for subgroups $G\leq \mathrm{S}_m\wr\mathrm{S}_n$ containing $\mathrm{A}_m^{[n]}$.

math.GR↗

arXiv2023

The proportion of non-degenerate complementary subspaces in classical spaces

Given positive integers $e_1,e_2$, let $X_i$ denote the set of $e_i$-dimensional subspaces of a fixed finite vector space $V=(\mathbb{F}_q)^{e_1+e_2}$. Let $Y_i$ be a non-empty subset of $X_i$ and let $α_i=|Y_i|/|X_i|$. We give a positive lower bound, depending only on $α_1,α_2,e_1,e_2,q$, for the proportion of pairs $(S_1,S_2)\in Y_1\times Y_2$ which intersect trivially. As an application, we bound the proportion of pairs of non-degenerate subspaces of complementary dimensions in a finite classical space that intersect trivially. This problem is motivated by an algorithm for recognizing classical groups. By using techniques from algebraic graph theory, we are able to handle orthogonal groups over the field of order 2, a case which had eluded Niemeyer, Praeger, and the first author.

math.CO↗

arXiv2022

Tournaments and Even Graphs are Equinumerous

A graph is called odd if there is an orientation of its edges and an automorphism that reverses the sense of an odd number of its edges, and even otherwise. Pontus von Brömssen (né Andersson) showed that the existence of such an automorphism is independent of the orientation, and considered the question of counting pairwise non-isomorphic even graphs. Based on computational evidence, he made the rather surprising conjecture that the number of pairwise non-isomorphic even graphs on $n$ vertices is equal to the number of pairwise non-isomorphic tournaments on $n$ vertices. We prove this conjecture using a counting argument with several applications of the Cauchy-Frobenius Theorem.

math.CO↗

arXiv2022

Random generation of direct sums of finite non-degenerate subspaces

Let $V$ be a $d$-dimensional vector space over a finite field $\mathbb{F}$ equipped with a non-degenerate hermitian, alternating, or quadratic form. Suppose $|\mathbb{F}|=q^2$ if $V$ is hermitian, and $|\mathbb{F}|=q$ otherwise. Given integers $e, e'$ such that $e+e'\leqslant d$, we estimate the proportion of pairs $(U, U')$, where $U$ is a non-degenerate $e$-subspace of $V$ and $U'$ is a non-degenerate $e'$-subspace of $V$, such that $U\cap U'=0$ and $U\oplus U'$ is non-degenerate (the sum $U\oplus U'$ is direct and usually not perpendicular). The proportion is shown to be positive and at least $1-c/q>0$ for some constant $c$. For example, $c=7/4$ suffices in both the unitary and symplectic cases. The arguments in the orthogonal case are delicate and assume that $\dim(U)$ and $\dim(U')$ are even, an assumption relevant for an algorithmic application (which we discuss) for recognising finite classical groups. We also describe how recognising a classical groups $G$ relies on a connection between certain pairs $(U,U')$ of non-degenerate subspaces and certain pairs $(g,g')\in G^2$ of group elements where $U={\rm im}(g-1)$ and $U'={\rm im}(g'-1)$.

math.GR↗

arXiv2022

The probability of spanning a classical space by two non-degenerate subspaces of complementary dimension

Let $n,n'$ be positive integers and let $V$ be an $(n+n')$-dimensional vector space over a finite field $\mathbb{F}$ equipped with a non-degenerate alternating, hermitian or quadratic form. We estimate the proportion of pairs $(U, U')$, where $U$ is a non-degenerate $n$-subspace and $U'$ is a non-degenerate $n'$-subspace of $V$, such that $U+ U'=V$ (usually such spaces $U$ and $U'$ are not perpendicular). The proportion is shown to be at least $1-c/|\mathbb{F}|$ for some constant $c\leqslant 2$ in the symplectic or unitary cases, and $c<3$ in the orthogonal case.

math.GR↗

arXiv2022

On the maximum of the weighted binomial sum $2^{-r}\sum_{i=0}^r\binom{m}{i}$

The weighted binomial sum $f_m(r)=2^{-r}\sum_{i=0}^r\binom{m}{i}$ arises in coding theory and information theory. We prove that,for $m\not \in\{0,3,6,9,12\}$, the maximum value of $f_m(r)$ with $0\leqslant r\leqslant m$ occurs when $r=\lfloor m/3\rfloor+1$. We also show this maximum value is asymptotic to $\frac{3}{\sqrt{πm}}\left(\frac{3}{2}\right)^m$ as $m\to\infty$.

math.CO↗

arXiv2022

The groups $G$ satisfying a functional equation $f(xk) = xf(x)$ for some $k \in G$

We study the groups $G$ with the curious property that there exists an element $k\in G$ and a function $f\colon G\to G$ such that $f(xk)=xf(x)$ holds for all $x\in G$. This property arose from the study of near-rings and input-output automata on groups. We call a group with this property a $J$-group. Finite $J$-groups must have odd order, and hence are solvable. We prove that every finite nilpotent group of odd order is a $J$-group if its nilpotency class $c$ satisfies $c\le6$. If $G$ is a finite $p$-group, with $p>2$ and $p^2>2c-1$, then we prove that $G$ is $J$-group. Finally, if $p>2$ and $G$ is a regular $p$-group or, more generally, a power-closed one (i.e., in each section and for each $m\geq1$ the subset of $p^m$-th powers is a subgroup), then we prove that $G$ is a $J$-group.

math.GR↗

arXiv2021

Sequences of linear codes where the rate times distance grows rapidly

For a linear code $C$ of length $n$ with dimension $k$ and minimum distance $d$, it is desirable that the quantity $kd/n$ is large. Given an arbitrary field $\mathbb{F}$, we introduce a novel, but elementary, construction that produces a recursively defined sequence of $\mathbb{F}$-linear codes $C_1,C_2, C_3, \dots$ with parameters $[n_i, k_i, d_i]$ such that $k_id_i/n_i$ grows quickly in the sense that $k_id_i/n_i>\sqrt{k_i}-1>2i-1$. Another example of quick growth comes from a certain subsequence of Reed-Muller codes. Here the field is $\mathbb{F}=\mathbb{F}_2$ and $k_i d_i/n_i$ is asymptotic to $3n_i^{c}/\sqrt{π\log_2(n_i)}$ where $c=\log_2(3/2)\approx 0.585$.

cs.IT↗

arXiv2021

Most permutations power to a cycle of small prime length

We prove that most permutations of degree $n$ have some power which is a cycle of prime length approximately $\log n$. Explicitly, we show that for $n$ sufficiently large, the proportion of such elements is at least $1-5/\log\log n$ with the prime between $\log n$ and $(\log n)^{\log\log n}$. The proportion of even permutations with this property is at least $1-7/\log\log n$.

math.GR↗

arXiv2021

Modules induced from a normal subgroup of prime index

Let $G$ be a finite group and $H$ a normal subgroup of prime index $p$. Let $V$ be an irreducible ${\mathbb F}H$-module and $U$ a quotient of the induced ${\mathbb F}G$-module $V\kern-3pt\uparrow$. We describe the structure of $U$, which is semisimple when ${\rm char}({\mathbb F})\ne p$ and uniserial if ${\rm char}({\mathbb F})=p$. Furthermore, we describe the division rings arising as endomorphism algebras of the simple components of $U$. We use techniques from noncommutative ring theory to study ${\rm End}_{{\mathbb F}G}(V\kern-3pt\uparrow)$ and relate the right ideal structure of ${\rm End}_{{\mathbb F}G}(V\kern-3pt\uparrow)$ to the submodule structure of $V\kern-3pt\uparrow$.

math.RT↗

arXiv2020

Subgroups of Classical Groups that are Transitive on Subspaces

For each finite classical group $G$, we classify the subgroups of $G$ which act transitively on a $G$-invariant set of subspaces of the natural module, where the subspaces are either totally isotropic or nondegenerate. Our proof uses the classification of the maximal factorisations of almost simple groups. As a first application of these results we classify all point-transitive subgroups of automorphisms of finite thick generalised quadrangles.

math.GR↗

arXiv2020

Point-primitive generalised hexagons and octagons and projective linear groups

We discuss recent progress on the problem of classifying point-primitive generalised polygons. In the case of generalised hexagons and generalised octagons, this has reduced the problem to primitive actions of almost simple groups of Lie type. To illustrate how the natural geometry of these groups may be used in this study, we show that if $\mathcal{S}$ is a finite thick generalised hexagon or octagon with $G \leqslant{\rm Aut}(\mathcal{S})$ acting point-primitively and the socle of $G$ isomorphic to ${\rm PSL}_n(q)$ where $n \geqslant 2$, then the stabiliser of a point acts irreducibly on the natural module. We describe a strategy to prove that such a generalised hexagon or octagon $\mathcal{S}$ does not exist.

math.GR↗