Dirac-type results for tilings and coverings in ordered graphs

Abstract

A recent paper of Balogh, Li and Treglown initiated the study of Dirac-type problems for ordered graphs. In this paper we prove a number of results in this area. In particular, we determine asymptotically the minimum degree threshold for forcing (i) a perfect $H$-tiling in an ordered graph, for any fixed ordered graph $H$ of interval chromatic number at least $3$; (ii) an $H$-tiling in an ordered graph $G$ covering a fixed proportion of the vertices of $G$ (for any fixed ordered graph $H$); (iii) an $H$-cover in an ordered graph (for any fixed ordered graph $H$). The first two of these results resolve questions of Balogh, Li and Treglown whilst (iii) resolves a question of Falgas-Ravry. Note that (i) combined with a result of Balogh, Li and Treglown completely determines the asymptotic minimum degree threshold for forcing a perfect $H$-tiling. Additionally, we prove a result that combined with a theorem of Balogh, Li and Treglown, asymptotically determines the minimum degree threshold for forcing an almost perfect $H$-tiling in an ordered graph (for any fixed ordered graph $H$). Our work therefore provides ordered graph analogues of the seminal tiling theorems of K\"uhn and Osthus [Combinatorica 2009] and of Koml\'os [Combinatorica 2000]. Each of our results exhibits some curious, and perhaps unexpected, behaviour. Our solution to (i) makes use of a novel absorbing argument.