On the essential dimension of cyclic p-groups THANKSREF="*" ID="*"The author gratefully acknowledges support from the Swiss National Science Fundation, grant no. 200020-109174/1 (project leader: E. Bayer-Fluckiger)

Let p be a prime number, let K be a field of characteristic not p, containing the p-th roots of unity, and let r≥1 be an integer. We compute the essential dimension of ℤ/p r ℤ over K (Theorem 4.1). In particular, i) We have edℚ(ℤ/8ℤ)=4, a result which was conjectured by Buhler and Reichstein in 1995 (unpublished). ii) We have edℚ(ℤ/p r ℤ)≥p r-1.