On a conjecture concerning dyadic oriented matroids.
On a conjecture concerning the Petersen graph.
On a conjecture of Andrews.
On a conjecture of Frankl and Füredi.
On a conjecture of M. E. Watkins on graphical regular representations of finite groups
On a conjecture of quintas and arc-traceability in upset tournaments
A digraph D = (V,A) is arc-traceable if for each arc xy in A, xy lies on a directed path containing all the vertices of V, i.e., hamiltonian path. We prove a conjecture of Quintas [7]: if D is arc-traceable, then the condensation of D is a directed path. We show that the converse of this conjecture is false by providing an example of an upset tournament which is not arc-traceable. We then give a characterization for upset tournaments in terms of their score sequences, characterize which arcs of...
On a conjecture of Sárközy and Szemerédi
Two infinite sequences A and B of non-negative integers are called infinite additive complements if their sum contains all sufficiently large integers. In 1994, Sárközy and Szemerédi conjectured that there exist infinite additive complements A and B with lim sup A(x)B(x)/x ≤ 1 and A(x)B(x)-x = O(minA(x),B(x)), where A(x) and B(x) are the counting functions of A and B, respectively. We prove that, for infinite additive complements A and B, if lim sup A(x)B(x)/x ≤ 1, then, for any given M > 1,...
On a conjecture of the semigroup of fully indecomposable relations
On a conjecture of Turán
On a conjecture on the diameter of line graphs of graphs of diameter two
On a connection of number theory with graph theory
We assign to each positive integer a digraph whose set of vertices is and for which there is a directed edge from to if . We establish necessary and sufficient conditions for the existence of isolated fixed points. We also examine when the digraph is semiregular. Moreover, we present simple conditions for the number of components and length of cycles. Two new necessary and sufficient conditions for the compositeness of Fermat numbers are also introduced.
On a construction of regular Hadamard matrices
We give a construction for regular Hadamard matrices of order where is the order of a Hadamard matrix and is the order of a regular Hadamard matrix. The construction can be used to construct regular Hadamard matrices with special properties and includes several constructions which have been given previously. In the final section we consider the case in more detail.
On a covering problem of Mullin and Stanton for binary matroids.
On a discrete Dido-type question.
On a discrete version of the antipodal theorem
The classical theorem of Borsuk and Ulam [2] says that for any continuous mapping there exists a point such that f(-x) = f(x). In this note a discrete version of the antipodal theorem is proved in which is replaced by the set of vertices of a high-dimensional cube equipped with Hamming’s metric. In place of equality we obtain some optimal estimates of which were previously known (as far as the author knows) only for f linear (cf. [1]).
On a distance of groups and Latin squares
On a facet of the balanced subgraph polytope
On a family of cubic graphs containing the flower snarks
We consider cubic graphs formed with k ≥ 2 disjoint claws (0 ≤ i ≤ k-1) such that for every integer i modulo k the three vertices of degree 1 of are joined to the three vertices of degree 1 of and joined to the three vertices of degree 1 of . Denote by the vertex of degree 3 of and by T the set . In such a way we construct three distinct graphs, namely FS(1,k), FS(2,k) and FS(3,k). The graph FS(j,k) (j ∈ 1,2,3) is the graph where the set of vertices induce j cycles (note that the graphs...
On a family of generalized Pascal triangles defined by exponential Riordan arrays.
On a four-colour theorem.