Counterexamples to a Conjecture of Dombi in Additive Number Theory

Abstract: We disprove a 2002 conjecture of Dombi from additive number theory. More precisely, we find examples of sets $A \subset \mathbb{N}$ with the property that $\mathbb{N} \setminus A$ is infinite, but the sequence $n \rightarrow |{ (a,b,c) \, : \, n=a+b+c \text{ and } a,b,c \in A }|$, counting the number of $3$-compositions using elements of $A$ only, is strictly increasing.