A connected open set in
Euclidean space is convex if it is locally supported at each boundary point; indeed,
the same statement holds in any complete Riemannian manifold for which all
geodesics are minimal. On the other hand, in an arbitrary complete n-dimensional
Riemannian manifold M the question, under what circumstances global convexity
properties are implied by local ones, involves the notion of cut locus. This question
will be considered here.