Allocca et al.'s three-reflection bound for dihedral groups

Let DnD_n be the dihedral group, with order-nn cyclic subgroup generated by the usual rotation. Let SS be a generating set composed of three involutions, none of which belongs to this cyclic subgroup. For gotinDng otin D_n and sotinSs otin S—wait, the quantity needed here is defined by

λ1(G,S):=maxgG, sS{lS(gsg1)}.\lambda_{1}(G,S):=\max_{g\in G,\ s\in S}\{l_S(gsg^{-1})\}.

Here lS(g)l_S(g) is the minimal number of elements of SS whose product is gg. Allocca et al.'s conjecture. For every such generating set,

λ1(Dn,S)n2+1.\lambda_{1}(D_n,S)\leq\left\lfloor\frac{n}{2}\right\rfloor+1.

This conjecture extends the previously proved existence result for a suitable three-reflection generating set to all three-reflection generating sets. The source does not state whether the conjecture has been resolved.

Sources & referencesView supporting material

Primary source

Michael Allocca and Max Trimmer, “Perturbation Effects on Word Lengths in Three-Reflection Symmetric Presentations of Dihedral Groups”, arXiv:2506.19216 (2025).

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