Majority Edge Colouring of Hypergraph

Abstract

Motivated by recent work on majority edge-colourings of graphs, we initiate the study of the corresponding problem for hypergraphs. First, sharpening the probabilistic argument by a $KL$ large-deviation estimate, we obtain a sufficient minimum-degree condition of order $k^3\log(kr)$ with the sharp large-deviation constant $ I_k:=D\!\left(\frac1k\middle\|\frac1{k+1}\right)=\Theta(k^{-3}), $ where $D(\cdot\|\cdot)$ denotes the binary relative entropy. Our main constructive result shows that every hypergraph of rank at most $r$ and minimum degree at least $2rk^2$ admits a $1/k$-majority $(k+1)$-edge-colouring. The proof is based on a hypergraph extension of the key discrepancy lemma used in the graph case. We also show that the logarithmic dependence on the rank can be determined asymptotically. If $\mu_k(r)$ denotes the least minimum-degree threshold that guarantees a $1/k$-majority $(k+1)$-edge-colouring for all hypergraphs of rank at most $r$, then for every fixed $k\ge2$, $ \mu_k(r)=\frac{\log r}{I_k}+O_k(\log\log r). $ In particular, the correct logarithmic threshold is of order $k^3\log r$. Finally, we determine the correct order of the degree--colour trade-off. For integers $k\ge2$, $p\ge1$, and $r\ge2$, let $\nu_{k,p}(r)$ denote the least integer $q$ such that every hypergraph of rank at most $r$ and minimum degree at least $kp$ admits a $1/k$-majority $q$-edge-colouring. Then $ \nu_{k,p}(r)=\Theta_{k,p}(r^{1/p}). $ In particular, minimum degree at least $k^2-k$ guarantees a $1/k$-majority $O_k(r^{1/(k-1)})$-edge-colouring, and this exponent is best possible.