Ehresmann's lemma

In mathematics, Ehresmann's lemma or Ehresmann's fibration theorem states that a smooth mapping

where M and N are smooth manifolds, such that

1. f is a surjective submersion, and
2. f is a proper map, (in particular if M is compact)

is a locally trivial fibration. This is a foundational result in differential topology, and exists in many further variants. It is due to Charles Ehresmann.