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I am trying to plot eigenvalues of a matrix. I get some errors like “precision may be lost”. As a result, there are some unwanted jumps in the plot. Any suggestion of what am I doing wrong?

Thanks

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You are running into numerical precision trouble. Use exact numbers instead. With all your definitions, change the calculation of eigenvalues to:

eigenvalues = Eigenvalues[Rationalize[H, 0]]

You can then plot them as you expected, although the following naïve plot is remarkably slow:

Plot[
  Evaluate@N@eigenvalues, {bx, -0.2, 0.2}, 
  Frame -> True, FrameLabel -> {"B", "y"}
]

eigenvalues - no zero crossings

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  • $\begingroup$ Thank you very much for your help. Would you please explain briefly what does "Evaluate@N@" do in the last line? I could not google it. $\endgroup$
    – sara sam
    Commented Jan 4, 2021 at 23:47
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    $\begingroup$ @sarasam The results from Eigenvalues are symbolic and the plotting was very slow. N@eigenvalues turns them into machine-precision entities. Because of the Hold attribute of Plot, I added Evaluate to force pre-evaluation of its argument, and make sure that Plot was dealing with the machine precision argument. I am not sure that it made a great difference in terms of plotting speed. You could remove it and see how king it takes to plot, I.e. Plot[eigenvalues, ...]. $\endgroup$
    – MarcoB
    Commented Jan 5, 2021 at 14:27

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