A logarithmic approximation of linearly-ordered colourings

Abstract

A linearly ordered (LO) -colouring of a hypergraph assigns to each vertex a colour from the set {0, 1, . . . , − 1} in such a way that each hyperedge has a unique maximum element. Barto, Batistelli, and Berg conjectured that it is NP-hard to find an LO -colouring of an LO 2-colourable 3-uniform hypergraph for any constant ≥ 2 [STACS’21] but even the case = 3 is still open. Nakajima and Zivn ˇ y gave polynomial-time algorithms for ´ finding, given an LO 2-colourable 3-uniform hypergraph, an LO colouring with ∗ ( √ ) colours [ICALP’22] and an LO colouring with ∗ ( 3 √ ) colours [ACM ToCT’23]. Very recently, Louis, Newman, and Ray gave an SDP-based algorithm with ∗ ( 5 √ ) colours. We present two simple polynomial-time algorithms that find an LO colouring with (log2 ()) colours, which is an exponential improvement

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