Cycle obstruction conjecture for ground-graph-independent blowup Ramsey numbers

Let rNr\in\mathbb{N} and let HH be a graph containing a cycle. Write GrHG\stackrel{r}{\to}H when every rr-colouring of the edges of GG contains a monochromatic copy of HH, and let H[t]H[t] denote the tt-blowup of HH.

Cycle obstruction conjecture. There exists an integer tt such that, for every ss, there is a graph GG satisfying

GrHbutG[s]̸rH[t].G\stackrel{r}{\to}H\quad\text{but}\quad G[s]\not\stackrel{r}{\to}H[t].

Together with the proved ground-graph-independent behaviour for forests, this conjecture would classify forests as exactly the finite graphs whose blowup Ramsey numbers do not depend on the ground graph. The source establishes the required obstruction for 3-chromatically connected graphs, while the assertion for every graph containing a cycle remains open.

Sources & referencesView supporting material

Primary source

António Girão and Robert Hancock, “Two Ramsey problems in blowups of graphs”, arXiv:2205.12826 (2024).

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