Minimizing Monochromatic Subgraphs of $K_{n,n}$

Given any $r$-edge coloring of $K_{n,n}$, how large is the maximum (over all $r$ colors) sized monochromatic subgraph guaranteed to be? We give answers to this problem for $r \leq 8$, when $r$ is a perfect square, and when $r$ is one less than a perfect square all up to a constant additive term that depends on $r$. We give a lower bound on this quantity that holds for all $r$ and is sharp when $r$ is a perfect square up to a constant additive term that depends on $r$. Finally, we give a construction for all $r$ which provides an upper bound on this quantity up to a constant additive term that depends on $r$, and which we conjecture is also a lower bound.

💡 Research Summary

The paper investigates a natural extremal problem on complete bipartite graphs. For a given integer r ≥ 1, consider any r‑edge‑coloring of the complete bipartite graph Kₙ,ₙ. For each color i, let the set of vertices “touched” by i be those incident to at least one edge of color i. Define

 g(n,r) = min_{colorings f:E(Kₙ,ₙ)→