Problems to Chapter 8

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### Problems to Chapter 8

Problem 1.

1. Write Laplace equation in spherical coordinates.
2. Look for a solution in the form $u=P(\rho)\Phi(\phi)\Theta(\theta)$.
3. Write equations and their solutions for $P$ and $\Theta$.
4. Write equation (with parameters $l$ and $m$) for $\Phi$; what values can take $(l,m)$?

Problem 2.

For equation to $\Phi$ find solutions in the form $L(\cos(\phi))$ where $L$ is a polynomial of degree $l$ with respect to $\cos(\phi), \sin(\phi)$ in the following cases:

1. $l=0$ and all possible values $m$;
2. $l=1$ and all possible values $m$;
3. $l=2$ and all possible values $m$;
4. $l=3$ and $m=0$.

Problem 3.

a. Solve \begin{align} &\Delta u=0 && x^2+y^2+z^2< 1,\label{8.P.1}\\ &u=g(x,y,z) && x^2+y^2+z^2=1 \label{8.P.2} \end{align} with $g(x,y,z)$ defined below.
Hint. If $g$ is a polynomial of degree $m$ look for \begin{equation} u=g - P(x,y,z)(x^2+y^2+z^2-R^2) \label{8.P.3} \end{equation} with $P$ a polynomial of degree $(m-2)$. Here $R$ is the radius of the ball. If $g$ has some rotational symmetry, so $P$ has.

b. Represent $u$ as a sum of homogeneous harmonic polynomials.

c. Plug $x=\rho \sin(\phi)\cos(\theta)$, $x=\rho \sin(\phi)\sin(\theta)$, $z=\rho \cos(\phi)$.

1. $g=x^2+y^2-z^2$.
2. $g=z(x^2+y^2)$.
3. $g=xyz$.
4. $g=x^4+y^4+z^4$.