I am reading a proof and one line says "As $M$ has no boundary..." . To the best of my knowledge, all we know about $M$ is that it is a compact semi-Riemannian manifold with constant (sectional) curvature.
I was quite puzzled and starting wondering whether it was due to the constant curvature, but I didn't really get anywhere.
Do the before mentioned properties of $M$ imply that it doesn't have a boundary, or could it simply be that when not explicitly saying $M$ a manifold with boundary we assume $M$ not to have a boundary?