All Lambda-Designs With λ = 2 p are Type-1

A-design is a family B ₁,B ₂,...,B _v of subsets of X={1, 2,..., v} such that B _i B _j = for all i jand not all B _i are of the same size. Ryser's andWoodall's -design conjecture states thateach -design can be obtained from a symmetricblock design by a certain complementation procedure. Our mainresult is that the conjecture is true when is twice a prime number.     

A black-box group algorithm for recognizing finite symmetric and alternating groups, I

We present a Las Vegas algorithm which, for a given black-box group known to be isomorphic to a symmetric or alternating group, produces an explicit isomorphism with the standard permutation representation of the group. This algorithm has applications in computations with matrix groups and permutation groups.

In this paper, we handle the case when the degree of the standard permutation representation is part of the input. In a sequel, we shall treat the case when the value of is not known in advance.

As an important ingredient in the theoretical basis for the algorithm, we prove the following result about the orders of elements of : the conditional probability that a random element is an -cycle, given that , is at least .

     

The Number of Directions Determined by Points in the Three-Dimensional Euclidean Space

   Abstract. Let X be a set of n points in the three-dimensional Euclidean space such that no three points in X are on the same line and there is no plane containing all points in X . An old conjecture states that pairs of points in X determine at least 2n-3 directions. We prove the weaker result that X determines at least 1.75n-2 directions.     

On Hajnal's triangle-free game

We investigate the triangle-free game proposed by András Hajnal. Starting with the empty graph onn points, two players alternatingly pick edges. The loser is the player who is forced to select an edge which completes a triangle. We determine the winner in a version of the game with the additional rule that the chosen edges must always give a connected subgraph ofK _n . Some other versions are also investigated.     

The Primitive Permutation Groups of Squarefree Degree

The paper gives lists of all the primitive permutation groupsof squarefree degree. All such groups are either solvable andact on a prime number of points, or are almost simple. Amongthe almost simple examples, the groups of Lie type have rankat most 2, or the point stabilizer is a parabolic subgroup.2000 Mathematics Subject Classification 20B15.     

Square-Free Non-Cayley Numbers. On Vertex-Transitive Non-Cayley Graphs of Square-Free Order

A complete classification is given of finite primitive permutation groups which contain a regular subgroup of square-free order. Then a collection of square-free numbers n is obtained such that there exists a vertex-primitive non-Cayley graph on n vertices if and only if n is a member of .     

Large Families of Cospectral Graphs

Seidel switching is an operation on graphs G satisfyingcertain regularity properties so that the resulting graph Hhas the same spectrum as G. If G issimple then the complement of G and the complementof H are also cospectral. We use a generalizationof Seidel switching to construct exponentially large familiesof cospectral graphs with cospectral complements.     

Erdős–Ko–Rado and Hilton–Milner Type Theorems for Intersecting Chains in Posets

t -intersecting k-chains in posets using the kernel method. These results are common generalizations of the original EKR and HM theorems, and our earlier results for intersecting k-chains in the Boolean algebra. For intersecting k-chains in the c-truncated Boolean algebra we also prove an exact EKR type theorem (for all n) using the shift method. An application of the general theorem gives a similar result for t-intersecting chains if n is large enough. Received November 20, 1997     

Symmetrical path-cycle covers of a graph and polygonal graphs

A near-polygonal graph is a graph Γ which has a set C of m-cycles for some positive integer m such that each 2-path of Γ is contained in exactly one cycle in C. If m is the girth of Γ then the graph is called polygonal. We introduce a method for constructing near-polygonal graphs with 2-arc transitive automorphism groups. As special cases, we obtain several new infinite families of polygonal graphs.