12 1. INTRODUCTION
Chapter 11 picks up from Chapter 3 the discussion of conservation laws,
now systems of conservation laws. Unlike the general theoretical develop-
ments in Chapters 5–9, for which Sobolev spaces provide the proper abstract
framework, we are forced to employ here direct linear algebra and calculus
computations. We pay particular attention to the solution of Riemann’s
problem and to entropy criteria.
Chapter 12, an introduction to nonlinear wave equations, is new with
this edition. We provide long time and short time existence theorems for
certain quasilinear wave equations and an in-depth examination of semilinear
wave equations, especially for subcritical and critical power nonlinearities in
three space dimensions. To complement these existence theorems, the ﬁnal
section identiﬁes various criteria ensuring nonexistence of solutions.
Appendices A–E provide for the reader’s convenience some background
material, with selected proofs, on inequalities, linear functional analysis,
measure theory, etc.
The Bibliography is an updated and extensive listing of interesting PDE
books to consult for further information. Since this is a textbook and not
a reference monograph, I have mostly not attempted to track down and
document the original sources for the myriads of ideas and methods we will
encounter. The mathematical literature for partial diﬀerential equations is
truly vast, but the books cited in the Bibliography should at least provide
a starting point for locating the primary sources. (Citations to selected
research papers appear throughout the text.)
1. Classify each of the partial diﬀerential equations in §1.2 as follows:
(a) Is the PDE linear, semilinear, quasilinear or fully nonlinear?
(b) What is the order of the PDE?
2. Let k be a positive integer. Show that a smooth function deﬁned on
has in general
n + k − 1
n + k − 1
n − 1
distinct partial derivatives of order k.
(Hint: This is the number of ways of inserting n − 1 dividers | within
a row of k symbols ◦: for example, ◦ ◦ || ◦ ◦ ◦ | ◦ | ◦ ◦ ◦ || ◦ ◦ ◦ ◦|.
Explain why each such pattern corresponds to precisely one of the
partial derivatives of order k.)