A Cauchy-Davenport theorem for semigroups

Abstract

We generalize the Davenport transform and use it to prove that, for a (possibly non-commutative) cancellative semigroup $\mathbb A = (A, +)$ and non-empty subsets $X,Y$ of $A$ such that the subsemigroup generated by $Y$ is commutative, we have $|X + Y| \ge \min(\omega(Y), |X| + |Y| - 1)$, where $\omega(Y) := \sup_{y_0 \in Y \cap \mathbb A^{\times}} \inf_{y \in Y \setminus \{y_0\}} |<y - y_0>|$. This carries over the Cauchy-Davenport theorem to the broader setting of semigroups, and it implies, in particular, an extension of I. Chowla's and S.S. Pillai's theorems for cyclic groups and a notable strengthening of another generalization of the same Cauchy-Davenport theorem to commutative groups, where $\omega(Y)$ in the above is replaced by the minimal order of the non-trivial subgroups of $\mathbb A$.