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Ivancevic_Applied-Diff-Geom

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<strong>Applied</strong> Manifold <strong>Geom</strong>etry 2613.9.4 Application: Biophysical PDEsIn this subsection we consider two most important equations for biophysics:(1) The heat equation, which has been analyzed in muscular mechanicssince the early works of A.V. Hill ([Hill (1938)]); and(2) The Korteveg–de Vries equation, the basic equation for solitary modelsof muscular excitation–contraction dynamics.SupposeS : ∆ r (x, u (n) ) = 0,(r = 1, ..., l),is a system of DEs of maximal rank defined over M ⊂ X × U. If G is a localgroup of transformations acting on M, andpr (n) v[∆ r (x, u (n) )] = 0, whenever ∆(x, u (n) ) = 0, (3.79)(with r = 1, ..., l) for every infinitesimal generator v of G, then G is asymmetry group of the system S [Olver (1986)].3.9.4.1 The Heat EquationRecall that the (1 + 1)D heat equation (with the thermal diffusivity normalizedto unity)u t = u xx (3.80)has two independent variables x and t, and one dependent variable u, sop = 2 and q = 1. Equation (3.80) has the second–order, n = 2, and can beidentified with the linear submanifold M (2) ⊂ X × U (2) determined by thevanishing of ∆(x, t, u (2) ) = u t − u xx .Letv = ξ(x, t, u) ∂∂x + τ(x, t, u) ∂ ∂+ φ(x, t, u)∂t ∂ube a vector–field on X × U. According to (3.79) we need to now the secondprolongationpr (2) v = v + φ x ∂+ φ t ∂+ φ xx ∂+ φ xt ∂+ φ tt∂u x ∂u t ∂u xx ∂u xt∂∂u ttof v. Applying pr (2) v to (3.80) we find the infinitesimal criterion (3.79) tobeφ t = φ xx ,

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