Dombi's obstruction conjecture for strict monotonicity of higher-fold representation functions

Let ANA\subseteq\mathbb N and let h1h\geq 1 be an integer. Define the ordered hh-fold representation function by

RA,h(n)=#{(a1,,ah)Ah:a1++ah=n}.R_{A,h}(n)=\#\{(a_1,\ldots,a_h)\in A^h:a_1+\cdots+a_h=n\}.

The complement NA\mathbb N\setminus A is co-infinite when it is infinite. Dombi's conjecture. If NA\mathbb N\setminus A is infinite, then RA,h(n)R_{A,h}(n) cannot be eventually strictly increasing. This conjecture proposes an obstruction to strict monotonicity for representation functions of sets with infinite complement; the provided text gives no resolution, while Dombi's earlier result shows that eventual non-strict increase can occur for suitable co-infinite sets when h>4h>4.

Sources & referencesView supporting material

Primary source

Csaba Sándor and Quan-Hui Yang, “On the Monotonicity of Higher-Fold Representation Functions”, arXiv:2606.28849 (2026).

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