# Problem set 1, PDE

## Problem 1

Let u be a $C^1$ function defined on a neighborhood of the domain $\Omega$ such that $\Delta u = 0$ in $\Omega$. Then

$\displaystyle \int_{\partial \Omega} \partial_\nu u \; d\sigma = 0.$

## Problem 2

If the power series $\sum a_\alpha x^\alpha$ converges at $x^0$, then it converges on the cube $|x_j| where $r = \min_i |x_i^0|$.

## Problem 3

If u satisfies the Cauchy problem

$\begin{cases} \partial_t^mu = G\big(x,t,(\partial_x^\alpha\partial_t^ju)_{|\alpha|+j\le m, j

then the functions  satisfy the system

$\partial_t y_{\alpha j} = y_{\alpha(j+1)} \quad |\alpha|+j < m,$                              $\partial_t y_{\alpha j} = \partial_{x_i}y_{(\alpha-e_i)(j+1)} \quad |\alpha|+j=m, j

$\displaystyle \partial_t y_{0m} = \frac{\partial G}{\partial t} + \sum_{|\alpha|+j

where $i = \min\{j: \alpha_j\not=0\}$, with data

$y_{\alpha j}(x,0) = \partial_x^\alpha \phi_j(x), \quad j                                          $y_{0m}(x,0) = G\big(x,0,(\partial_x^\alpha \phi_j(x))_{|\alpha|+j\le m, j

## Problem 4

Carry out the explicit calculations to reduce the equation $\Delta u = f$ to the first order system of the previous problem.

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