text: The Leray projection has the following properties:
  # The Leray projection is a projection:   for all .
  # The Leray projection is a divergence-free operator:  for all .
  # The Leray projection is simply the identity for the divergence-free vector fields:   for all  such that .
  # The Leray projection vanishes for the vector fields coming from a potential:   for all .