An obstacle in the use of Evans function theory for stability
analysis of traveling waves occurs when the spectrum of the
linearized operator about the wave accumulates at the imaginary
axis, since the Evans function has in general been constructed only
away from the essential spectrum. A notable case in which this
difficulty occurs is in the stability analysis of viscous shock
profiles. Here we prove a general theorem, the “gap lemma,”
concerning the analytic continuation of the Evans function
associated with the point spectrum of a traveling wave into the
essential spectrum of the wave. This allows geometric stability
theory to be applied in many cases where it could not be applied
previously.
We demonstrate the power of this method by analyzing the stability
of certain undercompressive viscous shock waves. A necessary
geometric condition for stability is determined in terms of the sign
of a certain Melnikov integral of the associated viscous profile.
This sign can easily be evaluated numerically. We also compute it
analytically for solutions of several important classes of systems.
In particular, we show for a wide class of systems that homoclinic
(solitary) waves are linearly unstable, confirming these as the
first known examples of unstable viscous shock waves. We also show
that (strong) heteroclinic undercompressive waves are sometimes
unstable. Similar stability conditions are also derived for Lax and
overcompressive shocks and for n × n conservation laws, n ≥ 2.
© 1998 John Wiley & Sons, Inc.