Wreath products of the groups of monotone permutations

We introduce the concept of wreath product of the m-groups of permutations and prove that an m-transitive group of permutations with an m-congruence is embeddable into the wreath product of the suitable m-transitive m-groups of permutations. This implies that an arbitrary m-transitive group in the product of two varieties of m-groups embeds into the wreath product of the suitable m-transitive groups of these varieties.     

6-Transitive graphs

A connected graph is n-transitive if, whenever two n-tuples are isometric, there is an automorphism mapping the first to the second. It is shown that a 6-transitive graph is complete multipartite, or complete bipartite with a matching deleted, or a cycle, or one of three special graphs on 9, 12 and 20 vertices. These graphs are n-transitive for all n; but there are graphs (the smallest on 56 vertices) which are 5- but not 6-transitive.     

Abelian groups of monotonic permutations

We study m-transitive representations of Abelian m-groups. Representations are found which mimic a variety A {\mathcal A} of all Abelian m-groups and a variety J {\mathcal J} of m-groups defined by an identity x _*  = x ⁻¹.     

Über symmetrische Graphen vom grad fünf

Let G be a connected 1-transitive graph of valency five. It is shown that the order of a vertex stabilizer divides 5 · 3² · 2¹⁷. A theorem of A. Gardiner bounding the order of a vertex stabilizer of a 2-transitive graph of valency 1 + p,p prime, is reproved.     

Benz planes of Dembowski type and groups of tangential projectivities

Using the tangential relation we introduce in Benz planes M of Dembowski type, which generalize the Benz planes over algebras of characteristic 2, the group ?? of tangential perspectivities. We prove that these groups have the same behaviour as the classical groups of projectivities if any tangential perspectivity is induced by an automorphism of M. As permutation groups of a circle onto itself the groups ?? essentially differs from the classical groups of projectivities. If M is a Laguerre plane of Dembowski type, then ?? is always sharply 3-transitive. For Minkowski planes of Dembowski type ?? is at least 2-transitive. If M is a finite Benz plane of order 2 ^s , then ?? is isomorphic to the group PGL ₂(2 ^s ) in its sharply 3-transitive representation.     

Automorphisms of graphs and coverings

We apply the theory of covering spaces to show how one can construct infinitely many finite s-transitive or locally s-transitive graphs. N. Biggs has used for similar purpose a special graph covering construction due to J. H. Conway.     

Hexavalent (G,s)-transitive graphs

Let X be a finite simple undirected graph with a subgroup G of the full automorphism group Aut(X). Then X is said to be (G, s)-transitive for a positive integer s, if G is transitive on s-arcs but not on (s + 1)-arcs, and s-transitive if it is (Aut(X), s)-transitive. Let G _v be a stabilizer of a vertex v ∈ V (X) in G. Up to now, the structures of vertex stabilizers G _v of cubic, tetravalent or pentavalent (G, s)-transitive graphs are known. Thus, in this paper, we give the structure of the vertex stabilizers G _v of connected hexavalent (G, s)-transitive graphs.     

The tame dimension vectors for trees

We consider representations of quivers over an algebraically closed field K. A dimension vector of a quiver is called hypercritical, if there is an m-parameter family of indecomposable representations for the dimension vector with m?2, but every family of representations for all smaller dimension vectors depends on a single parameter. We characterise the hypercritical dimension vectors for trees via their Tits forms and those of their decompositions and present the complete list of the hypercritical dimension vectors.Finally, this leads to a combinatorial classification of the tame dimension vectors for trees which is also given by the Tits forms.     

Symmetry groups of some perfect 1-factorizations of complete graphs

The following problem has arisen in the study of graphs, lattices and finite topologies. Is there a 1-factorization of K_2m the complete graph on 2n points, such that the union of every pair of distinct 1-factors is a hamiltonian circuit? In this paper it is noted that on $\text{K}{}_{\mathrm{2m}}\text{1?n?5}$, there is, up to relabelling, only one 1-factorization of the required type. On K₁₂ and whenever there are odd primes p,q>3 such that p + 1 = 2q, there are at least two different such 1-factorizations. These results are obtained by computing symmetry groups. The symmetry groups obtained are Frobenius groups of maximal order (i.e., sharply 2-transitive groups) and direct products of these groups with the group of order 2.     

A Classification of Countable Lower 1-transitive Linear Orders

This paper contains a classification of countable lower 1-transitive linear orders. This is the first step in the classification of countable 1-transitive trees given in Chicot and Truss (2009): the notion of lower 1-transitivity generalises that of 1-transitivity for linear orders, and it is essential for the structure theory of 1-transitive trees. The classification is given in terms of coding trees, which describe how a linear order is fabricated from simpler pieces using concatenations, lexicographic products and other kinds of construction. We define coding trees and show that a coding tree can be constructed from a lower 1-transitive linear order \((X, \leqslant )\) by examining all the invariant partitions on X. Then we show that a lower 1-transitive linear order can be recovered from a coding tree up to isomorphism.     

Tetravalent s-Transitive Graphs of Order 4p 2

Let s be a positive integer. A graph is s -transitive if its automorphism group is transitive on s-arcs but not on (s?+?1)-arcs. Let p be a prime. Zhou (Discrete Math 309:6081?C6086, 2009) classified tetravalent s-transitive graphs of order 4p. In this article a complete classification of tetravalent s-transitive graphs of order 4p ² is given.     

Sums of triangular numbers from the Frobenius determinant

We show that the denominator formula for the strange series of affine superalgebras, conjectured by Kac and Wakimoto and proved by Zagier, follows from a classical determinant evaluation of Frobenius. As a limit case, we obtain exact formulas for the number of representations of an arbitrary number as a sum of 4m²/d triangles, whenever d|2m, and 4m(m+1)/d triangles, when d|2m or d|2m+2. This extends recent results of Getz and Mahlburg, Milne, and Zagier.     

Semisymmetric graphs from polytopes

Every finite, self-dual, regular (or chiral) 4-polytope of type {3,q,3} has a trivalent 3-transitive (or 2-transitive) medial layer graph. Here, by dropping self-duality, we obtain a construction for semisymmetric trivalent graphs (which are edge- but not vertex-transitive). In particular, the Gray graph arises as the medial layer graph of a certain universal locally toroidal regular 4-polytope.     

Extension of Multiply Transitive Permutation Sets

Let G be a k-transitive permutation set on E and let E* = E∪{∞},∞ ? E; if G* is a (k: + 1)-transitive permutation set on E*, G* is said to be an extension of G whenever G _∝ ^* =G. In this work we deal with the problem of extending (sharply) k- transitive permutation sets into (sharply) (k + 1)-transitive permutation sets. In particular we give sufficient conditions for the extension of such sets; these conditions can be reduced to a unique one (which is a necessary condition too) whenever the considered set is a group. Furthermore we establish necessary and sufficient conditions for a sharply k- transitive permutation set (k ≥ 3) to be a group. Math. Subj. Class.: 20B20 Multiply finite transitive permutation groups 20B22 Multiply infinite transitive permutation groups     

A characterization of classical Minkowski planes over a perfect field of characteristic two

We give a new set of axioms defining the concept of (B^*)-plane (i.e. Minkowski plane without the tangency property) and we show that every (B^*)-plane in which a condition similar to the “Fano condition” of Heise and Karzel (see [5, § 3]) holds, is a Minkowski plane over a perfect field of characteristic two. In particular, every finite (B^*)-plane of even order is a Minkowski plane over a field. Consequences for strictly 3-transitive groups are derived from the preceding results; in particular, every strictly 3-transitive set of permutations of odd degree containing the identity is a protective group PGL₂(GF(2 ⁿ )) over a finite field GF(2 ⁿ , for some positive integer n.     

Examples of rank 3 product action transitive decompositions

A transitive decomposition is a pair where Γ is a graph and is a partition of the arc set of Γ such that there is a subgroup of automorphisms of Γ which leaves invariant and transitively permutes the parts in . In an earlier paper we gave a characterisation of G-transitive decompositions where Γ is the graph product K _m × K _m and G is a rank 3 group of product action type. This characterisation showed that every such decomposition arose from a 2-transitive decomposition of K _m via one of two general constructions. Here we use results of Sibley to give an explicit classification of those which arise from 2-transitive edge-decompositions of K _m .      

Definable functions in the ℵ<Subscript>0</Subscript>-categorical weakly circularly minimal structures

We continue studying the analogs of o-minimality and weak o-minimality for circularly ordered sets. We present a complete characterization of the behavior of unary definable functions in an ?₀-categorical 1-transitive weakly circularly minimal structure. Using it, we describe the ?₀-categorical 1-transitive nonprimitive weakly circularly minimal structures of convexity rank greater than 1 up to binarity.     

On the sharply 1-transitive subsets of PGL(2,pm)

If p is an odd prime and R is a sharply 1-transitive subset of PGL(2,p^m) which contains the identity but is not a group, then the subgroup generated by R is either PSL(2,p^m) or PGL(2,p^m).work done within the activity of G.N.S.A.G.A. and supported by the Italian Ministry of Public EducationDedicated to Professor Helmut Karzel on his 60^th birthday     

On sharply 2-transitive groups with generalized finite elements

We prove Jordan’s Theorem for infinite sharply 2-transitive groups satisfying the finiteness (a, b)-condition, with |a| · |b| even.