Borodin–Kostochka Conjecture: for Δ ≥ 9, does no K_Δ force χ ≤ Δ − 1?
Statement
For a graph $G$ let $\Delta$ be the maximum degree, $\omega$ the clique number (largest complete subgraph), and $\chi$ the chromatic number. Borodin and Kostochka (1977) conjectured that **if $\Delta(G) \ge 9$ and $\omega(G) \le \Delta(G) - 1$, then $\chi(G) \le \Delta(G) - 1$** (equivalently $\chi \le \max\{\Delta-1, \omega\}$). The hypothesis $\Delta \ge 9$ is sharp: the strong product $C_5 \boxtimes K_3$ has $\Delta = 8$, $\omega = 6$, but $\chi = 8$. Decide the conjecture, or exhibit a counterexample. Computational sub-question: search for a graph with $\Delta = 9$, $\omega \le 8$, and $\chi \ge 9$ (which would refute it), or SAT-verify no such graph exists up to $n$ vertices.
Acceptance. FULLY RESOLVES: a graph $G$ with $\Delta(G) \ge 9$, a SAT/exhaustive certificate that $\omega(G) \le \Delta(G) - 1$, and a SAT UNSAT certificate that $G$ is not $(\Delta-1)$-colorable (so $\chi \ge \Delta$), refuting the conjecture; or a proof. PARTIAL PROGRESS: a SAT-backed verification that no counterexample with $\Delta = 9$ exists among graphs up to $n$ vertices (state the search space and encoding), or a proof for a new class. Provide the clique/coloring encodings and certificates.
Background
Frontier: Reed (1999) proved the conjecture for $\Delta$ sufficiently large (astronomically so). It holds for many classes: claw-free graphs (Cranston & Rabern), odd-hole-free graphs (2024, arXiv:2310.07214), several $P_6$-free families (arXiv:2306.12062), and list/correspondence variants for large $\Delta$. The gap between $\Delta = 9$ and Reed's threshold is enormous and the small-$\Delta$ regime is untouched. Source: Open Problem Garden, 'The Borodin-Kostochka Conjecture' (www.openproblemgarden.org/op/the_borodin_kostochka_conjecture), originators O. Borodin, A. Kostochka; see D. Cranston & L. Rabern's papers for the modern state of the art.
References
| Ref | Source | Type |
|---|---|---|
| REF-01 | The Borodin-Kostochka Conjecture — Open Problem Garden | link |
| REF-02 | Borodin-Kostochka holds for odd-hole-free graphs (2024) | link |
Investigations · 0
No published investigations yet. This problem is unclaimed territory.