“…They appear to have been used first by McCarthy [19,20], in the context of acceleration wave propagation, and an independent derivation was give by Lax and Nelson [21]; see also Maugin [5], for example. The Lagrangian equations have an important role, in particular in the context of configurational forces (see, for example, [16], [22,23]), and have also led, at least in the static context, to rather elegant general formulations of the constitutive laws and the accompanying equilibrium equations for magnetoelastic [24] and electroelastic [25] solids capable of undergoing finite deformations. Such formulations have facilitated the solution of a number of boundary-value problems for both magnetoelasticity and electroelasticity (see, for example, [26,27,28]).…”