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Problems to Chapter 9

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Problems to Chapter 9

Problem 1.

Consider wave equation in dimension 2: \begin{equation} u_{tt}-\Delta u=0 \label{eq-9.P.1} \end{equation} in the disk {r\le a,\ 0\le \theta \le 2\pi}.

We consider solution in the form u=v(r,\theta)e^{i\omega t}. Then v satisfies Helmholtz equation \begin{equation} (\Delta +\omega^2)v=0. \label{eq-9.P.2} \end{equation}

Separate variables v=R(r)\Theta(\theta).

  1. Write down ODE, which should satisfy \Theta, and solve it (using periodicity).
  2. Write down ODE, which should satisfy R.

Problem 2.

Consider Laplace equation \Delta u=0 in the cylinder {r\le a,\ 0<z<b,\ 0\le \theta \le 2\pi}.

Separate variables u=R(r)Z(z)\Theta(\theta).

  1. Write down ODE, which should satisfy \Theta, and solve it (using periodicity).
  2. Write down ODE, which should satisfy Z, and solve it using Dirichlet boundary conditions on the top and bottom of the cylinder Z(0)=Z(b)=0.
  3. Write down ODE which should satisfy R.

Problem 3.

Consider wave equation (\ref{eq-9.P.1}) in the cylinder {r\le a,\ 0< z <h,\ 0\le \theta \le 2\pi}.

We consider solution in the form u=v(r,z,\theta)e^{i\omega t}. Then v satisfies Helmholtz equation (\ref{eq-9.P.2})

Separate variables v=R(r)Z(z)\Theta(\theta).

  1. Write down ODE which should satisfy \Theta and solve it (using periodicity).
  2. Write down ODE which should satisfy Z and solve it using homogeneous Dirichlet or Neumann conditions at z=0 and z=h.
  3. Write down ODE which should satisfy R.
  4. Be ready to consider other related domains: cut from the cylinder by restriction 0<\theta < \alpha and homogeneous Dirichlet or Neumann conditions at \theta=0 and \theta=\alpha.

Problem 4.

Consider wave equation (\ref{eq-9.P.1}) in the ball {\rho\le a,\ 0<\phi <\pi,\ 0\le \theta \le 2\pi}.

We consider solution in the form u=v(\rho,\phi,\theta)e^{i\omega t}. Then v satisfies Helmholtz equation (\ref{eq-9.P.2})

Separate variables v=P(\rho)\Phi(\phi)\Theta(\theta).

  1. Write down ODE, which should satisfy \Theta, and solve it (using periodicity).
  2. Write down ODE, which should satisfy \Phi.
  3. Write down, ODE which should satisfy P.

Hint. In the spherical coordinates \begin{equation*} \Delta u= u_{\rho\rho}+2\rho^{-1}u_\rho + \rho^{-2}\bigl(\Phi'' +\cot (\phi)\Phi'\bigr) + \rho^{-2}\sin^{-2}(\phi)u_{\theta\theta} \end{equation*}

Remark. "Solve" everywhere means "write down solution without justification".


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