I would like to solve $$\alpha u_{tt} = -\nabla^2u$$ with $\frac{\partial u}{\partial n} = 1$. On using a Galerkin approximation I obtain $$M\ddot{c}=\frac{1}{\alpha}(Dc+b)$$ where $M$ is the mass matrix and and $D$ is the stiffness matrix. I note that I can write this in the form $$\dot{U} = ZU+B$$

where $U = [c, \dot{c}]^T$ and $Z$ has off diagonal blocks top right: $I$ and bottom left $M^{-1}D$ and $B$ has only non-trivial entries in the "second half" of the vector given by $M^{-1}b$.

We can go ahead and use in implicit finite difference scheme on this problem then.(?)

I know my explanation is sparse but I am hoping that someone who might have solved this before can confirm whether this is a viable method. My solution is very "unstable" looking.

Perhaps I am being really naive using this method. Are there simple methods to solve this problem?

  • $\begingroup$ Do you have initial conditions? What is the geometry of your domain? $\endgroup$
    – nicoguaro
    Mar 10, 2018 at 22:51
  • 6
    $\begingroup$ The formulation you use is exactly the one I'm using here: dealii.org/8.5.0/doxygen/deal.II/step_23.html $\endgroup$ Mar 11, 2018 at 5:59
  • $\begingroup$ I am trying both a disk and a square. I am using Neumann boundary conditions with constant initial displacement and zero initial velocity(hence should remain constant). I think there must be something wrong with my implementation. Thanks for the link Wolfgang. $\endgroup$ Mar 11, 2018 at 7:59
  • $\begingroup$ Thanks, it all worked out. It was the classic case of missing minus sign. $\endgroup$ Mar 13, 2018 at 12:29
  • 1
    $\begingroup$ Great! You can answer your own question and accept your answer. It is considered good citizenship at SO. $\endgroup$
    – Anton Menshov
    Mar 17, 2018 at 2:04

1 Answer 1


The solution was to use


and to be vigilant of rogue minus signs.

  • $\begingroup$ Please do you still recall this question of yours? the link no longer works and I am looking for an answer $\endgroup$
    – NotaChoice
    May 19, 2022 at 15:46
  • $\begingroup$ Here's an updated link: dealii.org/current/doxygen/deal.II/step_23.html I think the issue was that dealii progressed beyond version 8.5.0. The link I included always references the current version. $\endgroup$ Jun 9, 2022 at 18:49

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