Optimal labelling schemes for adjacency, comparability, and reachability

We construct asymptotically optimal adjacency labelling schemes for every hereditary class containing $2^{\mathrm{\Omega }(n^2)}$ $n$-vertex graphs as $n\to \infty$. This regime contains many classes of interest, for instance perfect graphs or comparability graphs, for which we obtain an adjacency labelling scheme with labels of $n\mathrm{/}4+o(n)$ bits per vertex. This implies the existence of a reachability labelling scheme for digraphs with labels of $n\mathrm{/}4+o(n)$ bits per vertex and comparability labelling scheme for posets with labels of $n\mathrm{/}4+o(n)$ bits per element. All these results are best possible, up to the lower order term.