Graphon Colourings and Limits of Graph Partitions

Abstract

We develop a framework for pure and mixed graphon colourings and use it to study the limiting behaviour of graph partition parameters under cut convergence. Pure colourings correspond to measurable partitions, while mixed colourings assign a probability distribution on the colour set to each point. We develop the representation, equivalence and convergence theory, and show that every mixed graphon colouring has an equivalent pure representative, possibly on a different probability space. For each fixed finite colour set, we establish compactness and describe the topology of coloured graph limits using the cut norm on graphons and the weak-$*$ topology on colourings. The usual coloured cut distance characterises convergence for pure colourings, but not for general mixed colourings. As applications, we prove that the Cheeger constant and modularity converge to their mixed graphon counterparts along cut-convergent graph sequences whose orders tend to infinity, assuming uniformly linear minimum degree and positive limiting edge density, respectively. Both mixed functionals are invariant under graphon equivalence. We also establish their continuity in cut distance on the corresponding classes of graphons, with a stability bound for the Cheeger constant. For modularity, we further characterise coloured graphons attaining extremal modularity values and prove attainment of the mixed optimum for each fixed number of colours. We also exhibit a graphon whose modularity supremum is not attained by any finite colouring.