Odd-Ramsey numbers of Hamilton cycles

Abstract: The odd-Ramsey number $r_{\text odd}(n,H)$ of a graph $H$, as introduced by Alon in his work on graph-codes, is the minimum number of colours needed to edge-colour $K_n$ so that every copy of $H$ intersects some colour class in an odd number of edges. In this paper, we determine the odd-Ramsey number of Hamilton cycles up to a small multiplicative factor, proving that $r_{\text odd}(n,C_n) = Θ(\sqrt{n})$. Our upper bound follows from an explicit finite-field construction, while the matching lower bound uses a combinatorial framework based on parity switches. We also initiate the study of odd-Ramsey numbers of Hamilton cycles in Dirac graphs, demonstrating that a small increase in the minimum degree beyond $n/2$ forces nontrivial odd-Ramsey numbers.