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The energy principle is taken as the basis for studying of a toroidal equilibrium; this raises the problem of the minimum of the energy integral. This problem is studied in accordance with the methods of the theory of the calculus of variations. The stability can be deduced from the behaviour of solutions of an integro-differential system. The existence of a certain "admissible" solution — that is, one having a physical meaning — of zero value on two magnetic surfaces, is a necessary and sufficient condition for instability.
M. Bineau (1962) studied this question.