Induced Size-Ramsey Numbers of Odd Cycles via Spread Measures

Abstract

We prove that, for every $k\ge2$ and every sufficiently large odd $n$, the induced size--Ramsey number of $C_n$ satisfies \[ \widehat R_{\mathrm{ind}}(C_n;k)=e^{\Theta(k)}n. \] This improves the previous upper bound of $e^{O(k\log k)}n$ and resolves a conjecture of Brada\v{c}, Dragani\'c and Sudakov. We obtain this result by proving a stronger form of their odd-hole gadget conjecture: we construct a graph with $e^{O(k)}$ edges whose every $k$-edge-colouring contains a monochromatic induced odd cycle of length $O(\log k)$. Our proof develops a spread-based approach to finding induced cycles. The key idea is to place spread probability weights directly on families of candidate cycles and control their possible chords through a quadratic chord score. Combining this with random sparsification and hypergraph containers yields a sparse host graph that works for every $k$-edge-colouring.