A mathematical object partly named for this quantity is related to a harmonic function in Bochner’s formula. A type of this quantity is related to a Jacobi field inequality in the Rauch comparison theorem. Covariant derivatives of types of this quantity are related in the contracted Bianchi identities. The Killing-Hopf (15[1])theorem relates objects with a constant value of this quantity to a quotient of a space form by a group. (15[1])One type of this quantity is invariant under local isometry by the (*) Theorema Egregium. When this quantity is high, two geodesics from a point spread apart more slowly than when it is low. This quantity is characterised by the Einstein tensor in general (10[1])relativity, a theory where space-time has this property. For 10 points, name this quantity that may measure how much a surface deviates from being flat. ■END■