The constant momentum flux condition can be obtained by integrating the momentum equation across the jet.
where  is used to simplify the above equation. The mass flux  across any cross section normal to the  axis is not constant, because there is a slow entrainment of outer fluid into the jet, and it's a part of the boundary layer solution. This can be easily verified by integrating the continuity equation across the boundary layer.
where symmetry condition  is used.