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 $L(\cos(\phi),\sin(\phi))$ 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.

  1. Solve \begin{align} &\Delta u=0 && x^2+y^2+z^2< a^2,\label{8.P.1}\\ &u=g(x,y,z) && x^2+y^2+z^2=a^2 \label{8.P.2} \end{align} with $g(x,y,z)$ defined below and $a=1$.
    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-a^2) \label{8.P.3} \end{equation} with $P$ a polynomial of degree $(m-2)$. Here $a$ is the radius of the ball. If $g$ has some rotational or reflectional symmetry, so $P$ has.
  2. Represent $u$ as a sum of homogeneous harmonic polynomials.
  3. Plug $x=\rho \sin(\phi)\cos(\theta)$, $y=\rho \sin(\phi)\sin(\theta)$, $z=\rho \cos(\phi)$.

  4. $g=x^2+y^2-z^2$.
  5. $g=z(x^2+y^2)$.
  6. $g=xyz$.
  7. $g=x^4+y^4+z^4$.
  8. $g=x^2yz$.
  9. $g=xyz^2$.
  10. $g=x^2y^2$.
  11. $g=x^2z^2$.
  12. $g=xz^3$.
  13. $g=x^3z$.

Problem 4. Find function $u$, harmonic in $\{x^2+y^2+z^2\ge a^2\}$, decaying at infinity, and coinciding with $g(x,y,z)$ as $x^2+y^2+z^2=a^2$ with $g(x,y,z)$, defined in Problem 3 (a)-(j), and $a=1$.
Hint. Combine Problem 4 and Problem 7.P.6.

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