Constructions of Self-complementary Circulants with No Multiplicative Isomorphisms

The main topic of the paper is the question of the existence of self-complementary Cayley graphs Cay(G, S) with the property S^σ ≠ = G^# \ S for all σ Aut(G). We answer this question in the positive by constructing an infinite family of self-complementary circulants with this property. Moreover, we obtain a complete classification of primes p for which there exist self-complementary circulants of order p²with this property.     

A class of self-complementary vertex-transitive digraphs

We characterize the class of self-complementary vertex-transitive digraphs on a prime number p of vertices. Using this, we enumerate (i) self-complementary strongly vertex-transitive digraphs on p vertices, (ii) self-complementary vertex-transitive digraphs on p vertices, (iii) self-complementary vertex-transitive graphs on p vertices. Finally it is shown that every self-complementary vertex-transitive digraph on p vertices is either a tournament or a graph.     

Normal cirulant graphs with noncyclic regular subroups

We prove that any circulant graph of order n with connection set S such that n and the order of ?(S), the subgroup of ? that fixes S set‐wise, are relatively prime, is also a Cayley graph on some noncyclic group, and shows that the converse does not hold in general. In the special case of normal circulants whose order is not divisible by 4, we classify all such graphs that are also Cayley graphs of a noncyclic group, and show that the noncyclic group must be metacyclic, generated by two cyclic groups whose orders are relatively prime. We construct an infinite family of normal circulants whose order is divisible by 4 that are also normal Cayley graphs on dihedral and noncyclic abelian groups. © 2005 Wiley Periodicals, Inc. J Graph Theory     

A class of vertex-transitive digraphs,II

(1). We determine the number of non-isomorphic classes of self-complementary circulant digraphs with pq vertices, where p and q are distinct primes. The non-isomorphic classes of these circulant digraphs with pq vertices are enumerated. (2). We also determine the number of non-isomorphic classes of self-complementary, vertex-transitive digraphs with a prime number p vertices, and the number of self-complementary strongly vertex-transitive digraphs with p vertices. The non-isomorphic classes of strongly vertex-transitive digraphs with p vertices are also enumerated.     

Semisymmetric cubic graphs of order 16<Emphasis Type="Italic">p</Emphasis><Superscript>2</Superscript>

An undirected graph without isolated vertices is said to be semisymmetric if its full automorphism group acts transitively on its edge set but not on its vertex set. In this paper, we inquire the existence of connected semisymmetric cubic graphs of order 16p ². It is shown that for every odd prime p, there exists a semisymmetric cubic graph of order 16p ² and its structure is explicitly specified by giving the corresponding voltage rules generating the covering projections.     

Classification of Cyclic Steiner Quadruple Systems

The problem of classifying cyclic Steiner quadruple systems (CSQSs) is considered. A computational approach shows that the number of isomorphism classes of such designs with orders 26 and 28 is 52,170 and 1,028,387, respectively. It is further shown that CSQSs of order 2p, where p is a prime, are isomorphic iff they are multiplier equivalent. Moreover, no CSQSs of order less than or equal to 38 are isomorphic but not multiplier equivalent.     

Classifying cubic edge-transitive graphs of order 8<Emphasis Type="Italic">p</Emphasis>

A simple undirected graph is said to be semisymmetric if it is regular and edge-transitive but not vertex-transitive. Let p be a prime. It was shown by Folkman (J. Combin. Theory 3 (1967) 215–232) that a regular edge-transitive graph of order 2p or 2p ² is necessarily vertex-transitive. In this paper, an extension of his result in the case of cubic graphs is given. It is proved that, every cubic edge-transitive graph of order 8p is symmetric, and then all such graphs are classified.     

Half-Transitive Graphs of Prime-Cube Order

We call an undirected graph X half-transitive if the automorphism group Aut X of X acts transitively on the vertex set and edge set but not on the set of ordered pairs of adjacent vertices of X. In this paper we determine all half-transitive graphs of order p ³ and degree 4, where p is an odd prime; namely, we prove that all such graphs are Cayley graphs on the non-Abelian group of order p ³ and exponent p ², and up to isomorphism there are exactly (p – 1)/2 such graphs. As a byproduct, this proves the uniqueness of Holt's half-transitive graph with 27 vertices.     

PRODUCTS OF CIRCULANT GRAPHS

ABSTRACT

Graph products of circulants are studied. It is shown that if G and H are circulants and gcd(v(G), v(H)) = 1, then every B-product of G and H is again a circulant. We prove that if m ≠ 2, then the generalised prism K₂ ^mxC_n is a circulant iff n is odd. A similar result is deduced for the conjunction. We also prove that C_p x C_q is a circulant iff p and q are relatively prime. We close by showing that the composition of two circulants is again a circulant and explicitly describe the resultant circulant's jump sequence in terms of the constituent circulants' jump sequences.     

On ℤp-graded identities and cocharacters of the Grassmann algebra

Let p be an odd prime number, F a field of characteristic zero, and let Ebe the unitary Grassmann algebra generated by the infinite-dimensionalF-vector space L. We determine the bases of the ?_p-graded identities.Moreover we compute the ?_p-graded codimension and cocharacter sequences for the algebra E endowed with any ?_p-grading such that L is a homogeneous subspace.     

Complemented subgroups and <Emphasis Type="Italic">p</Emphasis>-nilpotence in finite groups

Let p be the smallest prime divisor of the order of a finite group G. We find sufficient conditions for G to be p-nilpotent based on the existence of complements in G for p-subgroups of certain orders. In particular, we generalize a recent result of M. Asaad.     

Two-dimensional flag-transitive planes revisited

This paper shows that the odd order two-dimensional flag-transitive planes constructed by Kantor-Suetake constitute the same family of planes as those constructed by Baker-Ebert. Moreover, for orders satisfying a modest number theoretical assumption this family consists of all possible such planes of that order. In particular, it is shown that the number of isomorphism classes of (non-Desarguesian) two-dimensional flag-transitive affine planes of order q ² is precisely (q–1)/2 when q is an odd prime and precisely (q–1)/2e when q=p ^e is an odd prime power with exponent e that is a power of 2. An enumeration is given in other cases that uses the Möbius inversion formula.This work was partially supported by NSA grant MDA 904-95-H-1013.This work was partially supported by NSA grant MDA 904-94-H-2033.     

The Influence of Hughes Type Action

We call the action of an automorphism α of a finite group G a Hughes type action if it is described by conditions on the orders of elements of G ? α ? ? G. In the present paper we study the structure of finite group G admitting an automorphism α of prime order p so that the orders of elements in G ? α ? ? G are not divisible by p ².     

Rank one quotients of vector groups

ABSTRACT

In this paper we study the possible torsion in even-dimensional higher class groups Cl _2n (Λ)(n ≥ 1) of an order Λ in a semisimple algebra A over a number field F with a ring of integers 𝒪 _F . We show that for certain orders, called generalized Eichler orders, bip-torsion in Cl _2n (Λ) can only occur for primes p dividing prime ideals ? of 𝒪 _F , at which Λ is not maximal. In particular, the results apply to Eichler orders in quaternion algebras and to hereditary orders.     

Constructions of Commutative Automorphic Loops

A loop whose inner mappings are automorphisms is an automorphic loop (or A-loop). We characterize commutative (A-)loops with middle nucleus of index 2 and solve the isomorphism problem. Using this characterization and certain central extensions based on trilinear forms, we construct several classes of commutative A-loops of order a power of 2. We initiate the classification of commutative A-loops of small orders and also of order p ³, where p is a prime.     

Syntomic regulators and special values of p-adic L-functions

In this paper p-adic analogs of the Lichtenbaum Conjectures are proven for abelian number fields F and odd prime numbers p, which generalize Leopoldt's p-adic class number formula, and express special values of p-adic L-functions in terms of orders of K-groups and higher p-adic regulators. The approach uses syntomic regulator maps, which are the p-adic equivalent of the Beilinson regulator maps. They can be compared with étale regulators via the Fontaine-Messing map, and computations of Bloch-Kato in the case that p is unramified in F lead to results about generalized Coates-Wiles homomorphisms and cyclotomic characters. Oblatum 14-V-96 & 9-X-97     

Vertex-transitive self-complementary uniform hypergraphs of prime order

For an integer n and a prime p, let . In this paper, we present a construction for vertex-transitive self-complementary k-uniform hypergraphs of order n for each integer n such that for every prime p, where ?=max{k₍₂₎,(k−1)₍₂₎}, and consequently we prove that the necessary conditions on the order of vertex-transitive self-complementary uniform hypergraphs of rank k=2^? or k=2^?+1 due to Potoňick and Šajna are sufficient. In addition, we use Burnside’s characterization of transitive groups of prime degree to characterize the structure of vertex-transitive self-complementary k-hypergraphs which have prime order p in the case where k=2^? or k=2^?+1 and , and we present an algorithm to generate all of these structures. We obtain a bound on the number of distinct vertex-transitive self-complementary graphs of prime order , up to isomorphism.     

Sylow subgraphs in self-complementary vertex transitive graphs

A graph is self-complementary if it is isomorphic to its complement. A graph is vertex transitive if for each choice of vertices u and $v$ there is an automorphism that carries the vertex u to $v$. The number of vertices in a self-complementary vertex-transitive graph must necessarily be congruent to 1 mod 4. However, Muzychuk has shown that if $p^m$ is the largest power of a prime p dividing the order of a self-complementary vertex-transitive graph, then $p^m$ must individually be congruent to 1 mod 4. This is accomplished by establishing the existence of a self-complementary vertex transitive subgraph of order $p^m$, a result reminiscent of the Sylow theorems. This article is a self-contained survey, culminating with a detailed proof of Muzychuk's result.     

On the Existence of a Noninner Automorphism of Order p for p-Groups

We obtain sufficient conditions for the existence of a noninner automorphism of order p for finite p-groups. We show that groups of order p ⁿ (n < 7, p is a prime number, p > 3) possess a noninner automorphism of order p.     

On Sylow Subgraphs of Vertex-Transitive Self-Complementary Graphs

One of the basic facts of group theory is that each finite groupcontains a Sylow p-subgroup for each prime p which divides theorder of the group. In this note we show that each vertex-transitiveself- complementary graph has an analogous property. As a consequenceof this fact, we obtain that each prime divisor p of the orderof a vertex-transitive self-complementary graph satisfies thecongruence p^m 1(mod 4), where p^m is the highest power of pwhich divides the order of the graph. 1991 Mathematics SubjectClassification 05C25, 20B25.