Schiffler Problem 6.3

For admissible parameters a,ba,b and any two lattice paths P,QD(a,b)P,Q\in\mathcal{D}(a,b), does equality of their Lagrange numbers imply that their associated band graphs are isomorphic? That is, is Lag(P)=Lag(Q)\operatorname{Lag}(P)=\operatorname{Lag}(Q) sufficient to conclude Band(P)Band(Q)\operatorname{Band}(P)\cong\operatorname{Band}(Q)? This is false: there exist P,QD(17,9)P,Q\in\mathcal{D}(17,9) such that Lag(P)=Lag(Q)\operatorname{Lag}(P)=\operatorname{Lag}(Q) but Band(P)≇Band(Q)\operatorname{Band}(P)\not\cong\operatorname{Band}(Q).

Progress summary

Solved

A new preprint gives a counterexample, showing that the proposed uniqueness claim is false.

Schiffler Problem 6.36.3 asks whether equal Lagrange numbers force band graphs to be isomorphic. A new explicit counterexample answers this question negatively.

August 2026 counterexample

The preprint constructs the counterexample in the class D(17,9)\mathrm{D}(17,9), proving that equal Lagrange numbers do not determine the band graph up to isomorphism.

Current status (as of August 2026): Schiffler Problem 6.36.3 is resolved negatively by an explicit counterexample; related classification questions remain open.

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