arXiv subjects

Gregory Cherlin

Publications and source records attributed to Gregory Cherlin.

11 recordsLinked to original sources

arXiv2024

Pairs of separably closed fields and exotic groups

We look at simple groups associated primarily with the general theory of Moufang buildings, and to analyze their relation to stability theory in the model theoretic sense. As it becomes quite technical in the details, a lengthy introduction surveys the developments at a less detailed level. The text, beginning from the second section, first deals with some model theoretic algebra of fields, followed by an extended study of three associated families of simple groups coming from the theory of Tits buildings, Moufang polygons, and Timmesfeld's theory of exotic analogs of SL_2. The field theoretic part is fundamental ({\S}2). The rest of the paper relates this to group theoretic constructions, with two sections surveying the consequences for the original Tits and Timmesfeld theory before concentrating on the more exotic groups associated with Moufang polygons. A good deal of the group theoretical material is expository, aimed to make the relevant structural information meaningful to those coming from the direction of model theory.

math.LO↗

arXiv2021

Homogeneity and related topics: An extended bibliography

We give a bibliography of works relating to homogeneous structures in the sense of Fraïssé, and related topics, mainly through 2016, with some narrow updating through 2021. We first give a list arranged by topics, with references to the main bibliogrphay, which follows.

math.LO↗

arXiv2021

Ramsey expansions of 3-hypertournaments

We study Ramsey expansions of certain homogeneous 3-hypertournaments. We show that they exhibit an interesting behaviour and, in one case, they seem not to submit to current gold-standard methods for obtaining Ramsey expansions. This makes these examples very interesting from the point of view of structural Ramsey theory as there is a large demand for novel examples.

math.CO↗

arXiv2014

Universal graphs with a forbidden subgraph: Block path solidity

Let C be a finite connected graph for which there is a countable universal C-free graph, and whose tree of blocks is a path. Then the blocks of C are complete. This generalizes a result of Furedi and Komjath, and fits naturally into a set of conjectures regarding the existence of countable C-free graphs, with C an arbitrary finite connected graph.

math.CO↗

arXiv2008

A generation theorem for groups of finite Morley rank

We deal with two forms of the "uniqueness cases" in the classification of large simple $K^*$-groups of finite Morley rank of odd type, where large means the $m_2(G)$ at least three. This substantially extends results known for even larger groups having \Prufer 2-rank at least three, to cover the two groups $\PSp_4$ and $\G_2$. With an eye towards distant developments, we carry out this analysis for $L^*$-groups which is substantially broader than the $K^*$ setting.

math.GR↗

arXiv2008

Semisimple torsion in groups of finite Morley rank

We prove several results about groups of finite Morley rank without unipotent p-torsion: p-torsion always occurs inside tori, Sylow p-subgroups are conjugate, and p is not the minimal prime divisor of our approximation to the ``Weyl group.'' These results are quickly finding extensive applications within the classification project.

math.LO↗

arXiv2007

Minimal connected simple groups of finite Morley rank with strongly embedded subgroups

We show that a minimal nonalgebraic simple groups of finite Morley rank has Prufer rank at most 2, and eliminates tameness from Cherlin and Jaligot's past work on minimal simple groups. The argument given here begins with the strongly embedded minimal simple configuration of Borovik, Burdges and Nesin. The 0-unipotence machinery of Burdges's thesis is used to analyze configurations involving nonabelian intersections of Borel subgroups. The number theoretic punchline of Cherlin and Jaligot has been replaced with a new genericity argument.

math.GR↗

arXiv2007

Involutions in groups of finite Morley rank of degenerate type

This article proves a version of the Feit-Thompson theorem for simple groups of finite Morley rank: a connected groups of finite Morley rank with a finite Sylow 2-subgroup has a trivial Sylow 2-subgroups.

math.LO↗

arXiv2007

Borovik-Poizat rank and stability

There is an axiomatic treatment of Morley rank in groups, due to Borovik and Poizat. These axioms form the basis of the algebraic treatment of groups of finite Morley rank which is common today. There are, however, ranked structures, i.e. structures on which a Borovik-Poizat rank function is defined, which are not $\aleph_0$-stable. Poizat raised the issue of the relationship between this notion of rank and stability theory in the following terms: ``un groupe de Borovik est une structure stable, alors qu'un univers rangé n'a aucune raison de l'être ...''. Nonetheless, we show that a ranked structure is superstable.

math.LO↗

arXiv2005

Universal graphs with a forbidden subtree

We show that the problem of the existence of universal graphs with specified forbidden subgraphs can be systematically reduced to certain critical cases by a simple pruning technique which simplifies the underlying structure of the forbidden graphs, viewed as trees of blocks. As an application, we characterize the trees T for which a universal countable T-free graph exists.

math.LO↗

arXiv1998

Universal graphs with forbidden subgraphs and algebraic closure

We apply model theoretic methods to the problem of existence of countable universal graphs with finitely many forbidden connected subgraphs. We show that to a large extent the question reduces to one of local finiteness of an associated''algebraic closure'' operator. The main applications are new examples of universal graphs with forbidden subgraphs and simplified treatments of some previously known cases.

math.LO↗