We give a stability result for sparse convolutions on 2 (G) × 1 (G) for torsion-free discrete Abelian groups G such as Z. It turns out, that the torsion-free property prevents full cancellation in the convolution of sparse sequences and hence allows to establish stability in each entry, that is, for any fixed entry of the convolution the resulting linear map is injective with an universal lower norm bound, which only depends on the support cardinalities of the sequences. This can be seen as a reverse statement of the famous Young inequality for sparse convolutions. Our result hinges on a compression argument in additive set theory.
I. INTRODUCTIONAdditive problems have increasingly become a focus in combinatorics, number theory, group theory and Fourier analysis as pointed out, e.g., in the textbook of Tao and Vu [24]. The key hereby is an understanding of the additive structure of finite subsets of an Abelian group G. The main result in the herein presented work is the application of a recent compression result in additive set theory by Grynkiewicz [6, Theorem 20.10] to sparse convolutions on discrete Abelian groups which are torsionfree, i.e., for any N ∈ N and g ∈ G it holds N g = g + ⋅ ⋅ ⋅ + g = 0 if and only if g = 0. Compressing the convolution of sparse sequences, i.e., sequences with finite support sets, reduces to a compression of the sumset of their supports, since supp(x * y) ⊆ supp(x)+supp(y). Our compression result allows to obtain a reverse statement of Young's inequality [28,27] for the convolution of all sparse (x, y) ∈ 2 (G)×