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## Problem 1

Classify the following second-order PDE.

1. $\displaystyle \sum_{i=1}^n \partial^2_{x_i x_i}u + \sum_{1\le i<j\le n} \partial^2_{x_i x_j}u = 0$

2. $\displaystyle \sum_{1\le i<j \le n} \partial^2_{x_i x_j}u = 0$

## Problem 2

Use energy methods to discuss the uniqueness of the problem

$\begin{cases} \Delta u - u^3 = f & \text{in }\Omega\\u = \phi & \text{on }\partial\Omega.\end{cases}$

## Problem 3

Let $\Omega$ be a bounded $C^1$-domain in $\R^n$ and u be a $C^2$-function in $\bar\Omega\times[0,T]$ satisfying

$\begin{cases} u_t - \Delta u = f & \text{in }\Omega\times(0,\infty)\\ u(\cdot, 0) = u_0 & \text{in }\Omega\\ u=0 & \text{on }\partial\Omega\times(0,\infty).\end{cases}$

Then

$\displaystyle \sup_{0\le t \le T} \int_\Omega |\nabla u(\cdot,t)|^2 dx + \int_0^T\int_\Omega |\partial_t u|^2 dx dt \le C\Big( \int_\Omega |\nabla u_0|^2 dx + \int_0^T\int_\Omega |f|^2 dxdt \Big),$

where C is a positive constant depending only on $\Omega$.

## Problem 4

Verify the identity

$\displaystyle \frac{1}{2\pi} + \frac{1}{\pi} \sum_{k=1}^\infty r^k \cos k(\theta - \eta) = \frac{1}{2\pi} \frac{1 - r^2}{1 - 2r\cos(\theta - \eta) + r^2}.$