Twin-Width is Linear in the Poset Width

TL;DR

This paper proves that the twin-width of a poset is linearly bounded by its width, providing a direct and tight bound, and offers algorithms for constructing contraction sequences.

Contribution

It establishes a tight linear bound between poset width and twin-width, improving previous indirect bounds and providing constructive algorithms.

Findings

Posets of width d have twin-width at most 9d.

Posets of width 2 have twin-width exactly 2 in the worst case.

The paper provides algorithms to construct contraction sequences for posets.

Abstract

Twin-width is a new parameter informally measuring how diverse are the neighbourhoods of the graph vertices, and it extends also to other binary relational structures, e.g. to digraphs and posets. It was introduced just very recently, in 2020 by Bonnet, Kim, Thomasse and Watrigant. One of the core results of these authors is that FO model checking on graph classes of bounded twin-width is in FPT. With that result, they also claimed that posets of bounded width have bounded twin-width, thus capturing prior result on FO model checking of posets of bounded width in FPT. However, their translation from poset width to twin-width was indirect and giving only a very loose double-exponential bound. We prove that posets of width d have twin-width at most 9d with a direct and elegant argument, and show that this bound is asymptotically tight. Specially, for posets of width 2 we prove that in the worst case their twin-width is also equal 2. These two theoretical results are complemented with straightforward algorithms to construct the respective contraction sequence for a given poset.