← All papers
First page of Lean-verified lower bounds for the Shannon capacity of odd cycles

Lean-verified lower bounds for the Shannon capacity of odd cycles

Pjotr Buys, Sven Polak, Jeroen Zuiddam

math.CO Jul 31, 2026 · v1 cs.IT
Base valid tuples and their validity proofs for odd-cycle Shannon capacity lower bounds are formalised and verified in Lean.
We give new lower bounds for the Shannon capacities of small odd cycles: $Θ(C_7)\geq3.258805369885\ldots$, $Θ(C_{11})\geq5.294502522149\ldots$, $Θ(C_{13})\geq6.302455083464\ldots$, $Θ(C_{15})\geq7.301600534487\ldots$, $Θ(C_{19})\geq9.357192705918\ldots$, $Θ(C_{21})\geq10.342455853338\ldots$, and $Θ(C_{23})\geq11.328224257774\ldots$. The bounds are obtained by an iterative procedure due to Gao (2026) which is based on a method by Itty, Rosin, Carstensen and Reichman (2026). The bounds are fully formalised in Lean.

The Shannon capacity of odd cycles of length at least seven is unknown, and improving rigorous lower bounds requires large explicit independent sets in strong powers whose correctness is hard to verify by hand.

An iterative star-product procedure on valid tuples (Gao's gadget construction) is used to build large independent sets in strong powers of odd cycles from computationally discovered base tuples. Base tuples for C7, C11, C13, C15, C19, C21, C23 were obtained using large language models. The base tuples and their validity proofs are encoded as explicit literals in Lean, which checks that each is valid.

New lower bounds are established, e.g. Θ(C7) ≥ 3.258805..., Θ(C11) ≥ 5.294502..., and Θ(C13) ≥ 6.302455..., improving prior bounds; all bounds are fully formalised and verified in Lean.

npowernew boundpreviousϑ(C_n)
72003.25880533.25878913.3176672
111745.29450255.28977365.3863029
134326.30245506.30010916.4041685
19163849.35719279.35712009.4347713
23204811.328224211.327837911.4461936
Lower bounds on Θ(C_n): strong power, new bound, previous bound, and Lovász theta bound.