Three principal invariants with respect to this quantity are named for Chern-Pontryagin, Euler, and Kretschmann. A renormalization method named for this quantity is used to find the order parameter of topological phase transitions. Non-zero values of this quantity necessitate generalizing the derivative operator to the (*) covariant derivative. The affine connections vanish (10[1])for systems in which this quantity is equal to zero. The contracted Bianchi identity relates a tensor and a scalar describing this property, both of which are named for Ricci. Non-zero (10[1])values (10[2])of this quantity cause gravitational lensing, or the bending of light. When this quantity is non-zero, straight lines are generalized into geodesics. For 10 points, what quantity is zero for flat spacetime? ■END■