Geometric invariants for $p$-groups of class 2 and exponent $p$
We introduce geometric invariants for $p$-groups of class $2$ and exponent $p$. We report on their effectiveness in distinguishing among 5-generator $p$-groups of this type.
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arXiv subjects
E. A. O'Brien
Publications and source records attributed to E. A. O'Brien.
Elusive groups from non-split extensions
A finite transitive permutation group is elusive if it contains no derangements of prime order. These groups are closely related to a longstanding open problem in algebraic graph theory known as the Polycirculant Conjecture, which asserts that no elusive group is $2$-closed. Existing constructions of elusive groups mostly arise from split extensions. In this paper, we initiate the construction of elusive groups via non-split extensions. As a demonstration, we construct elusive groups of new degrees, namely $p^{3k-4}(p+1)/2$ for each Mersenne prime $p\geq7$ and integer $k\geq2$. We also construct the first examples of elusive groups with odd degree, namely $3^{k+1}\cdot5^2$, and twice odd degree, namely $2\cdot3^{k + 1}\cdot5^2$ for each $k\geq1$. We conclude by proposing further problems to advance this new direction of research.
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Categorification of characteristic structures
We develop a representation theory of categories as a means to explore characteristic structures in algebra. Characteristic structures play a critical role in isomorphism testing of groups and algebras, and their construction and description often rely on specific knowledge of the parent object and its automorphisms. In many cases, questions of reproducibility and comparison arise. Here we present a categorical framework that addresses these questions. We prove that every characteristic structure is the image of a functor equipped with a natural transformation. This shifts the local description in the parent object to a global one in the ambient category. Through constructions in representation theory, such as tensor products, we can combine characteristic structure across multiple categories. Our results are constructive, stated in the language of a constructive type theory, which facilitates implementations in theorem checkers.
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Conjugacy in finite classical groups
Let $G$ be a classical group defined over a finite field. We consider the following fundamental problems concerning conjugacy in $G$: 1. List a representative for each conjugacy class of $G$. 2. Given $x \in G$, describe the centralizer of $x$ in $G$, by giving its group structure and a generating set. 3. Given $x,y \in G$, establish whether $x$ and $y$ are conjugate in $G$ and, if so, then find explicit $z \in G$ such that $z^{-1}xz = y$. We present comprehensive theoretical solutions to all three problems, and use our solutions to formulate practical algorithms. In parallel to our theoretical work, we have developed in Magma complete implementations of our algorithms. They form a critical component of various general algorithms in computational group theory - for example, computing character tables and solving conjugacy problems in arbitrary finite groups.
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Exceptional groups of order $p^6$ for primes $p\geq 5$
The minimal faithful permutation degree $μ(G)$ of a finite group $G$ is the least integer $n$ such that $G$ is isomorphic to a subgroup of the symmetric group $S_n$. If $G$ has a normal subgroup $N$ such that $μ(G/N) > μ(G)$, then $G$ is exceptional. We prove that the proportion of exceptional groups of order $p^6$ for primes $p \geq 5$ is asymptotically 0. We identify $(11p+107)/2$ such groups and conjecture that there are no others.
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Presentations for the groups of order $p^6$ for prime $p\geq 7$
We list those presentations for the groups of order $p^6$ for prime $p\geq 7$ which are distributed as part of the {\sc SmallGroups} library.
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Comment on: The groups of order $p^6$ ($p$ an odd prime) By Rodney James, Math. Comput. 34 (1980), 613-637
Over the years various errors have been identified in the 1980 paper of James on the groups of order $p^6$, where $p$ is an odd prime. Here we summarise them.
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Minimal degrees for faithful permutation representations of groups of order $p^6$ where $p$ is an odd prime
We determine the minimal degree of a faithful permutation representation for each group of order $p^6$ where $p$ is an odd prime. We also record how to obtain such a representation.
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Local systems and Suzuki groups
We study geometric monodromy groups $G_{\geo,\sF_q}$ of the local systems $\sF_q$ on the affine line over $\F_2$ of rank $D=\sqrt{q}(q-1)$, $q=2^{2n+1}$, constructed in \cite{Ka-ERS}. The main result of the paper shows that $G_{\geo,\sF_q}$ is either the Suzuki simple group $\tw2 B_2(q)$, or the special linear group $\SL_D$. We also show that $\sF_8$ has geometric monodromy group $\tw2B_2(8)$, and arithmetic monodromy group $\Aut(\tw2 B_2(8))$ over $\F_2$, thus establishing \cite[Conjecture 2.2]{Ka-ERS} in full in the case $q=8$.
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Isomorphism invariant metrics
Within a category $\mathtt{C}$, having objects $\mathtt{C}_0$, it may be instructive to know not only that two objects are non-isomorphic, but also how far from being isomorphic they are. We introduce pseudo-metrics $d:\mathtt{C}_0 \times \mathtt{C}_0 \to [0,\infty]$ with the property that $x\cong y$ implies $d(x,y)=0$. We also give a canonical construction that associates to each isomorphism invariant a pseudo-metric satisfying that condition. This guarantees a large source of isomorphism invariant pseudo-metrics. We examine such pseudo-metrics for invariants in various categories.
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Classifying finite monomial linear groups of prime degree in characteristic zero
Let $p$ be a prime and let $\mathbb{C}$ be the complex field. We explicitly classify the finite solvable irreducible monomial subgroups of $\mathrm{GL}(p,\mathbb{C})$ up to conjugacy. That is, we give a complete and irredundant list of $\mathrm{GL}(p,\mathbb{C})$-conjugacy class representatives as generating sets of monomial matrices. Copious structural information about non-solvable finite irreducible monomial subgroups of $\mathrm{GL}(p,\mathbb{C})$ is also proved, enabling a classification of all such groups bar one family. We explain the obstacles in that exceptional case. For $p\leq 3$, we classify all finite irreducible subgroups of $\mathrm{GL}(p,\mathbb{C})$. Our classifications are available publicly in Magma.
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The $3$-closure of a solvable permutation group is solvable
Let $m$ be a positive integer and let $Ω$ be a finite set. The $m$-closure of $G\leq\operatorname{Sym}(Ω)$ is the largest permutation group on $Ω$ having the same orbits as $G$ in its induced action on the Cartesian product $Ω^m$. The $1$-closure and $2$-closure of a solvable permutation group need not be solvable. We prove that the $m$-closure of a solvable permutation group is always solvable for $m\geq3$.
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Thin subalgebras of Lie algebras of maximal class
For every field $F$ which has a quadratic extension $E$ we show there are non-metabelian infinite-dimensional thin graded Lie algebras all of whose homogeneous components, except the second one, have dimension $2$. We construct such Lie algebras as $F$-subalgebras of Lie algebras $M$ of maximal class over $E$. We characterise the thin Lie $F$-subalgebras of $M$ generated in degree $1$. Moreover we show that every thin Lie algebra $L$ whose ring of graded endomorphisms of degree zero of $L^3$ is a quadratic extension of $F$ can be obtained in this Lie algebra of maximal class over $E$ which are ideally $r$-constrained for a positive integer $r$.
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Presentations on standard generators for classical groups
For each family of finite classical groups, and their associated simple quotients, we provide an explicit presentation on a specific generating set of size at most 8. Since there exist efficient algorithms to construct this generating set in any copy of the group, our presentations can be used to verify claimed isomorphisms between representations of the classical group. The presentations are available in Magma.
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Deciding finiteness of matrix groups in positive characteristic
We present a new algorithm to decide finiteness of matrix groups defined over a field of positive characteristic. Together with previous work for groups in zero characteristic, this provides the first complete solution of the finiteness problem for finitely generated matrix groups over an arbitrary field. We also give an algorithm to compute the order of a finite matrix group over a function field of positive characteristic. Our MAGMA implementations of these algorithms are publicly available.
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Algorithms for the Tits alternative and related problems
We present an algorithm that decides whether a finitely generated linear group over an infinite field is solvable-by-finite: a computationally effective version of the Tits alternative. We also give algorithms to decide whether the group is nilpotent-by-finite, abelian-by-finite, or central-by-finite. Our algorithms have been implemented in MAGMA and are publicly available.
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Recognizing finite matrix groups over infinite fields
We present a uniform methodology for computing with finitely generated matrix groups over any infinite field. As one application, we completely solve the problem of deciding finiteness in this class of groups. We also present an algorithm that, given such a finite group as input, in practice successfully constructs an isomorphic copy over a finite field, and uses this copy to investigate the group's structure. Implementations of our algorithms are available in MAGMA.
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