Killing–Hopf theorem
Characterizes complete connected Riemannian manifolds of constant curvature
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In geometry, the Killing–Hopf theorem states that complete connected Riemannian manifolds of constant curvature are isometric to a quotient of a sphere, Euclidean space, or hyperbolic space by a group acting freely and properly discontinuously.[1] These manifolds are called space forms. The Killing–Hopf theorem was proved by Killing (1891) and Hopf (1926).