We classify non-symplectic automorphisms of odd prime order on irreducible holomorphic symplectic manifolds which are deformations of Hilbert schemes of any number n of points on K3 surfaces, extending results already known for n = 2. In order to do so, we study the properties of the invariant lattice of the automorphism (and its orthogonal complement) inside the second cohomology lattice of the manifold. We also explain how to construct automorphisms with fixed action on cohomology: in the cases n = 3, 4 the examples provided allow to realize all admissible actions in our classification. For n = 4, we present a construction of non-symplectic automorphisms on the Lehn-Lehn-Sorger-van Straten eightfold, which come from automorphisms of the underlying cubic fourfold.We remark that this is the first known geometric construction of a non-induced, non-symplectic automorphism of odd order on a manifold of K3 [4] -type. Moreover, thanks to it we are able to complete the list of examples of automorphisms of odd prime order p < 23 which realize all admissible pairs (T, S) for n = 3, 4.Acknowledgements. The authors thank Samuel Boissière, Andrea Cattaneo, Alice Garbagnati, Robert Laterveer and Giovanni Mongardi for many helpful discussions, as well as Christian Lehn and Manfred Lehn for their useful remarks and explanations. The authors are also extremely grateful to Alessandra Sarti and Bert van Geemen, for reading the paper and for their precious suggestions.
Preliminary notions2.1. Lattices. Definition 2.1. A lattice L is a free abelian group endowed with a symmetric, non-degenerate bilinear form (·, ·) : L × L → Z. The lattice is even if the associated quadratic form is even on all elements of L. If t is a positive integer, L(t) denotes the lattice having as bilinear form the one of L multiplied by t. Examples of lattices,