Improving the Erdős–Ginzburg–Ziv theorem for some non-abelian groups

Let G be a group of order m. Define s(G) to be the smallest value of t such that out of any t elements in G, there are m with product 1. The Erdős–Ginzburg–Ziv theorem gives the upper bound s(G)⩽2m−1, and a lower bound is given by s(G)⩾D(G)+m−1, where D(G) is Davenport's constant. A conjecture by Zhuang and Gao [J.J. Zhuang, W.D. Gao, Erdős–Ginzburg–Ziv theorem for dihedral groups of large prime index, European J. Combin. 26 (2005) 1053–1059] asserts that s(G)=D(G)+m−1, and Gao [W.D. Gao, A combinatorial problem on finite abelian groups, J. Number Theory 58 (1996) 100–103] has proven this for all abelian G. In this paper we verify the conjecture for a few classes of non-abelian groups: dihedral and dicyclic groups, and all non-abelian groups of order pq for p and q prime.