Miklós Ujvári In this paper after algebraical and geometrical preliminaries we present a Gram-Schmidt-type algorithmical conjecture, which if true settles the long-standing Hadamard conjecture concerning the existence of orthogonal matrices with elements of the same absolute value. CitationUnpublished, ELTE Operation Research Report 2015/1ArticleDownload View PDF
In the paper we consider degree, spectral, and semide finite bounds on the stability number of a graph. The bounds are obtained via reformulations and variants of the inverse theta function, a notion recently introduced by the author in a previous work. ArticleDownload View PDF
Tressler’s Theorem states that the long-standing Hadamard conjecture (concerning the existence of n by n orthogonal matrices with elements of the same absolute value, for n=4k, k=1,2,…) will be settled if we find n-2 pairwise orthogonal words in a hyperplane of words. In this paper we will prove the counterpart of Tressler’s Theorem: the existence … Read more
Recently the author introduced a semidefinite upper bound on the square of the stability number of a graph, the inverse theta number, which is proved here to be multiplicative with respect to the strong graph product, hence to be an upper bound for the square of the Shannon capacity of the graph. We also describe … Read more
In 1979, L. Lovasz defined the theta number, a spectral/semidefinite upper bound on the stability number of a graph, which has several remarkable properties (e.g. it is exact for perfect graphs). The definition of Lovasz’s theta number relies on the notion of orthonormal representation of the graph, and, dually, can be obtained by optimizing over … Read more
In the paper we consider degree, spectral, and semidefinite bounds on the stability and chromatic numbers of a graph: so-called weak sandwich theorems. We examine the additivity properties of the bounds (the sum of two graphs is their disjoint union), and as an application we tighten the bounds in the weak sandwich theorems, if the … Read more
In the paper, we describe various applications of the closedness and duality theorems of [7] and [8]. First, the strong separability of a polyhedron and a linear image of a convex set is characterized. Then,it is shown how stability conditions (known from the generalized Fenchel-Rockafellar duality theory) can be reformulated as closedness conditions. Finally, we … Read more
The Hadamard conjecture (unsolved since 1867) states that there exists an orthogonal matrix with entries of the same absolute value if and only if the order of the matrix is one, two, or is divisible by four. In the paper we reformulate this conjecture using Hurwitz-Radon word systems. (A Hurwitz-Radon word system is a system … Read more
In the seminal work [1] L. Lovász introduced the concept of an orthonormal representation of a graph, and also a related value, now popularly known as the Lováasz number of the graph. One of the remarkable properties of the Lovász number is that it lies sandwiched between the stability number and the complementer chromatic number. … Read more
Recently Cambini and Carosi described a characterization of pseudolinearity of quadratic fractional functions. A reformulation of their result was given by Rapcsák. Using this reformulation, in this paper we describe an alternative proof of the Cambini–Carosi Theorem. Our proof is shorter than the proof given by Cambini–Carosi and less involved than the proof given by … Read more