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I've spent far too much time coding and debugging multigrid. While I clearly can't post all of my code as it would be silly to ask someone to go through all that code, is there anything I should pay special attention to?

Note: I'm posting this because I don't want people to have to waste the time I had. This may not be completely relevant to computational science, but I thought it could be helpful for any beginners (as I certainly still am one).

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What language are you using? Would a simple working Python 1D multigrid code be of any use for comparison? –  David Ketcheson Feb 17 '13 at 19:16
    
@DavidKetcheson I'm using Matlab. There's so many ways to code MG though, so I don't know how comparable it would be. (Thank you though). My thoughts are that I'm missing a scaling factor somewhere. According to my norm plots, it runs for a few iterations, then the error just kinks like crazy. But the fact that it works for a few iterations suggests that I'm missing a scaling factor (or need to remove one). But I'm no expert; that's just my interpretation. –  AlanH Feb 18 '13 at 23:06
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2 Answers

up vote 4 down vote accepted

The smoother, restriction and prolongation can be tested independently. In essence, take a periodic domain, put in a particular Fourier mode and see if the code behaves in accordance with Fourier analysis. Also, in Briggs, Henson and McCormick's A Multigrid Tutorial you will find a section called Diagnostic tools (Chapter 4). That should help you with systematic debugging.

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We have collected a great deal of experience on how to debug finite element codes here: https://code.google.com/p/dealii/wiki/FrequentlyAskedQuestions

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