In geometry, an Hadamard space, named after Jacques Hadamard, is a non-linear generalization of a Hilbert space. In the literature they are also equivalently defined as complete CAT spaces. A Hadamard space is defined to be a nonempty complete metric spacesuch that, given any points x, y, there exists a point m such that for every point z, The point m is then the midpoint of x and y:. In a Hilbert space, the above inequality is equality, and in general an Hadamard space is said to be flat if the above inequality is equality. A flat Hadamard space is isomorphic to a closedconvex subset of a Hilbert space. In particular, a normed space is an Hadamard space if and only if it is a Hilbert space. The geometry of Hadamard spaces resembles that of Hilbert spaces, making it a natural setting for the study of rigidity theorems. In a Hadamard space, any two points can be joined by a unique geodesic between them; in particular, it is contractible. Quite generally, if B is a bounded subset of a metric space, then the center of the closed ball of the minimum radius containing it is called the circumcenter of B. Every bounded subset of a Hadamard space is contained in the smallest closed ball. If is the group of isometries of a Hadamard space leaving invariant B, then fixes the circumcenter of B. The basic result for a non-positively curved manifold is the Cartan–Hadamard theorem. The analog holds for a Hadamard space: a complete, connected metric space which is locally isometric to a Hadamard space has an Hadamard space as its universal cover. Its variant applies for non-positively curved orbifolds. Examples of Hadamard spaces are Hilbert spaces, the Poincaré disc, complete metric trees, -space with p, q ≥ 3 and 2pq ≥ p + q, and Hadamard manifolds, i.e. complete simply-connected Riemannian manifolds of nonpositive sectional curvature. Important examples of Hadamard manifolds are simply connected nonpositively curved symmetric spaces. Applications of Hadamard spaces are not restricted to geometry. In 1998, Dmitri Burago and Serge Ferleger used CAT geometry to solve a problem in Dynamical billiards: in a gas of hard balls, is there a uniform bound on the number of collisions? The solution begins by constructing a configuration space for the dynamical system, obtained by joining together copies of corresponding billiard table, which turns out to be an Hadamard space.
