Weberrsquo;s transformation is used to show how Linrsquo;s constraint should be replaced if fluid equations are derived from Hamiltonrsquo;s principle. The same technique is used to derive a threehyphen;circulation theorem and a generalization of Ertelrsquo;s theorem for perfect multifluid plasmas. The Hamiltonian and Lagrangian formulation of the equations for fluid and electromagnetic potentials is given, with a discussion of their multivaluedness and their gauge and time dependence for static magnetohydrodynamic equilibria. The linear stability of these equilibria is shown to depend on the weight of a single negative eigenvalue of the internal energy variation, compared with all other (positive) contributions to the lsquo;lsquo;energyrsquo;rsquo; functional.
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