where p = ∇u, q = ∇u, u= ∂u∕∂t, ü= ∂2u∕∂t2 represents, for suitable functions
W(p), V (q), a nonlinear hyperbolic equation with nonlinear viscosity and it appears
in models of nonlinear elasticity. In this paper existence and regularity of solutions
for the Cauchy problem will be established. In particular, if n = 2, or if n ≥ 3 and the
eigenvalues of (∂2V∕∂qj∂qj) belong to a “small” interval, then the solution is
classical. These results will actually be established for a system of equations of the
above type.