Cartan–Hadamard theorem

The Cartan-Hadamard theorem is a result in differential geometry, particularly concerning the geometry of Riemannian manifolds. It establishes conditions under which a complete Riemannian manifold without boundary is diffeomorphic to either the Euclidean space or has certain geometric properties related to curvature. Specifically, the theorem states that: If \( M \) is a complete, simply connected Riemannian manifold with non-positive sectional curvature (i.e.