General Relativity: An Introduction for Physicists · Problem 3.14
Vector calculus on manifolds — Hobson, Efstathiou & Lasenby
Physics · parallel-transport-geodesic-character
Problem
If the two vectors with contravariant components va and wa are each parallel-transported along a curve, show that vawa remains constant along the curve. Hence show that if a geodesic is timelike (or null or spacelike) at some point, it is timelike (or null or spacelike) at all points.