Covering grids with multiplicity

Given a finite grid in R², how many lines are needed to cover all but one point at least k times? Problems of this nature have been studied for decades, with a general lower bound having been established by Ball and Serra. We solve this problem for various types of grids, in particular showing the tightness of the Ball–Serra bound...

On the minimum degree of minimal Ramsey graphs for cliques versus cycles

A graph G is said to be q-Ramsey for a q-tuple of graphs (H₁,..., H_q), denoted by G →q (H₁,..., H_q), if every q-edge-coloring of G contains a monochromatic copy of Hi in color i for some i ε [q]. Let sq(H1,..., Hq) denote the smallest minimum degree of G over all graphs G that are minimal q-Ramsey...

Subspace coverings with multiplicities

We study the problem of determining the minimum number of affine subspaces of codimension that are required to cover all points of at least times while covering the origin at most times. The case is a classic result of Jamison, which was independently obtained by Brouwer and Schrijver for. The value of also follows from a well-known theorem...

The minimum degree of minimal Ramsey graphs for cliques

We prove that (Formula presented.), where (Formula presented.) is the Ramsey parameter introduced by Burr, Erdős and Lovász in 1976, which is defined as the smallest minimum degree of a graph (Formula presented.) such that any (Formula presented.) -colouring of the edges of (Formula presented.) contains a monochromatic (Formula presented.),...

Ryser's Conjecture for t-intersecting hypergraphs

A well-known conjecture, often attributed to Ryser, states that the cover number of an r-partite r-uniform hypergraph is at most r−1 times larger than its matching number. Despite considerable effort, particularly in the intersecting case, this conjecture remains wide open, motivating the pursuit of variants of the original conjecture....