In this paper, a supersymmetric extension of a system of hydrodynamic type equations involving Riemann invariants is formulated in terms of a superspace and superfield formalism. The symmetry properties of both the classical and supersymmetric versions of this hydrodynamical model are analyzed through the use of group-theoretical methods applied to partial differential equations involving both bosonic and fermionic variables. More specifically, we compute the Lie superalgebras of both models and perform classifications of their respective subalgebras. A systematic use of the subalgebra structures allow us to construct several classes of invariant solutions, including travelling waves, centered waves and solutions involving monomials, exponentials and radicals.Running Title: Supersymmetric system in Riemann invariants PACS: primary 02.20. Sv, 12.60.Jv, 02.30.Jr, Keywords: Riemann invariants, Lie superalgebra, invariant solutions. * email address: grundlan@crm.umontreal.ca † email address: hariton@crm.umontreal.ca
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I Introduction
A Historical BackgroundThe concept of differential invariants was first introduced by B. Riemann in 1858 in his classical works 1, 2 concerning the Euler equations for an ideal fluid flow in two independent variables u t + uu x + p ′ (ρ) ρ ρ x = 0, ρ t + uρ x + ρu x = 0, ρ > 0.Here, u is the local velocity of the fluid, the pressure p is assumed to be a differentiable function of the density ρ and p ′ = dp/dρ. Riemann investigated the formulation and mathematical correctness of problems involving the propagation and superposition of waves described by equation (1). Next, he constructed rank-2 solutions corresponding to the "superposition" of two waves propagating with local velocity u = ± (p ′ Riemann studied the asymptotic behavior of initial localized disturbances (i.e. initial data with compact support) corresponding to the above-mentioned waves. Consequently, he demonstrated that even for sufficiently small initial data, after some finite time T , these waves could be separated again in such a way that waves of the same type as those assumed in the initial data could be observed. Riemann noticed that the solution for systems of hydrodynamic type (3), even with arbitrarily smooth initial data, usually(could not be continued indefinitely in time. After a certain finite time T the solutions blew 2 up (more precisely, the first derivatives of the considered solution become unbounded after some finite time T > 0). So, for times t > T , smooth solutions to the Cauchy problem do not exist. This phenomenon is known as the gradient catastrophe. 3, 4 Furthermore, Riemann was interested in extending the solution in some more generalized sense beyond the time T of the blow up. On the basis of the conservation laws for the mass, energy and momentum, Riemann 2 and later Hugoniot 5 introduced the concept of weak solutions (non-continuous) in the form of shock waves. Based on this concept he proved some laws connecting the wave front velocity and parameters of the fluid state before and behi...