(0.1) At the end of the eighties, V. Berkovich developed in [2] and [3] a new theory of analytic geometry over non-archimedean fields, coming after those by Krasner, Tate [22] and Raynaud [20]. One of the main advantages of this approach is that the resulting spaces enjoy very nice topological properties: they are locally compact and locally pathwise connected (although nonarchimedean fields are totally disconnected, and often not locally compact). Moreover, Berkovich spaces have turned out to be "tame" objects -in the informal sense of Grothendieck's Esquisse d'un programme. Before illustrating this rather vague assertion by several examples, let us fix some terminology and notations.(0.2) Let k be a field which is complete with respect to a non-archimedean absolute value |.| and let k o be the valuation ring {x ∈ k, |x| 1}.(0.2.1) A (Berkovich) analytic space over k is a (locally compact, locally pathconnected) topological space X equipped with some extra-data, among which:-a sheaf of k-algebras whose sections are called analytic functions, which makes X a locally ringed space; -for every point x of X, a complete non-archimedean field (H (x), |.|) endowed with an isometric embedding k ֒→ H (x), and an "evaluation" morphism from O X,x to H (x), which is denoted by f → f (x).(0.2.2) To every k-scheme of finite type X is associated in a functorial way a k-analytic space X an , which is called its analytification ([3], §2.6); it is provided with a natural morphism of locally ringed spaces X an → X If f is a section of O X , we will often still denote by f its pull-back to O X an (X an ).Let us recall what the underlying topological space of X an is. As a set, it consists of couples (ξ, |.|) where ξ ∈ X and where |.| is a non-archimedean absolute value on κ(ξ) extending the given absolute value of k. If x = (ξ, |.|) is a point of X an , then H (x) is the completion of the valued field (κ(ξ), |.|)). The map X an → X is nothing but (ξ, |.|) → ξ; note that the pre-image of a * During this work the author was partially supported by ValCoMo (ANR-13-BS01-0006), and by a Rosi and Max Varon visiting professorship at Weizmann Institute.
1Zariski-open subset U of X on X an can be identified with U an . The topology X an is equipped with is the coarsest such than:• X an → X is continuous (otherwise said, U an is an open subset of X an for every Zariski-open subset U of X);• for every Zariski-open subset U of X and every f ∈ O X (U ), the map (ξ, |.|) → |f (ξ)| from U an to R + is continuous.Let us now assume that X is affine, say X = Spec A. The topological space underlying X an can then be given another description: it is the set of all multiplicative maps ϕ : A → R + that extend the absolute value of k and that satisfy the inequality ϕ(a + b) max(ϕ(a), ϕ(b)) for every (a, b) ∈ A 2 (such a map will be simply called a multiplicative semi-norm); its topology is the one inherited from the product topology on R A + . This is related as follows to the former description:• to any couple (ξ, |.|) as above corresponds the multiplica...