  Variational principles for magnetohydrodynamics were introduced by previous
authors both in Lagrangian and Eulerian form. In previous works [1] Yahalom &
Lynden-Bell and later Yahalom [2] introduced a simpler Eulerian variational
principle from which all the relevant equations of Magnetohydrodynamics can be
derived. The variational principles were given in terms of four independent
functions for non-stationary flows and three independent functions for
stationary flows. This is less than the seven variables which appear in the
standard equations of magnetohydrodynamics which are the magnetic field, the
velocity field and the density . In the case that the magnetohydrodynamic flow
has a non trivial topology such as when the magnetic lines are knotted or
magnetic and stream lines are knotted, some of the functions appearing in the
Lagrangian are non-single valued. Those functions play the same rule as the
phase in the Aharonov-Bohm celebrated effect [3].
