# Deriving a bound on a PDE

1. Sep 23, 2014

### physmatics

1. The problem statement, all variables and given/known data
Hi!
Not sure if I'm posting in the right section, this problem is from a course in scientific computing. Anyway, we're considering a set of PDEs:

$$u_t + Au_x = 0 \quad 0<x<1, \ t>0 \\ u(x,0) = f(x) \quad 0 \leq x \leq1 \\ u_1(0,t) = 0 \quad t \geq 0 \\ u_2(1,t) = 0 \quad t \geq 0 \\ u = \begin{pmatrix} u_1 \\ u_2 \end{pmatrix}, \quad A = \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}$$

Now, I want to compute a bound ||u(*, )|| in terms of f. We have the standard inner product and the norm, where ||u||^2 = (u,u).

3. The attempt at a solution
First and foremost, my idea is that I use what's called the energy method, where we multiply with u, integrate in space and apply the BCs, to find ||u||. What I can't wrap my head around though is the fact that A is a matrix and in reality I have two equations, but coupled since they both contain u1 and u2. Should I introduce two variables to multiply the equations with? How can I deal with the matrix while integrating? Or should I just give up the energy method completely and take on another approach?

Any help or small hints are much appreciated!

2. Sep 25, 2014

### Staff: Mentor

Hi Physmatics. You might find a better audience for your question in one of the mathematics homework forums. Maybe try
Calculus & Beyond Homework.

If you wish I can move your thread there. Let me know.

3. Oct 1, 2014