Nonlinear, hyperbolic systems of conservation laws such as the Euler e
quations of fluid dynamics admit a strict Riemann invariant, e.g. the
physical entropy. Here we discuss the structure of such systems, empha
sizing the effects of changes of variables, the existence of equivalen
t symmetric systems, and entropy conditions for discontinuities in the
form of entropy inequalities. (C) 1995 Academic Press, Inc.