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Editorial Team
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Asked: 2026-05-20T10:00:49+00:00

I try to extract coefficient from equations (system of equations) into list (matrix). I

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I try to extract coefficient from equations (system of equations) into list (matrix). I have tried CoefficientList[poly, {var1, var2, ...}] but without success.

This simple example should explain my problem:

Eq1 = a D[U[x1, x2], {x1, 2}] + b D[V[x1, x2], {x2, 2}]

reproduce problem

Any advice?

Edit:

Daniel’s Lichtblau solution is very clear (thanks you), but what if the equation that looks like this?

Eq1 = a D[U[x1, x2], {x1, 2}] + b D[V[x1, x2], {x2, 2}] + c W[x1, x2]

A simple example can be resolved as follows:

additional question

Is there any more elegant solution? (particularly for more complex expressions)

Ps I can’t understand why, but this solution gives me the correct result.

reproduce problem

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1 Answer

  1. Editorial Team

    Firstly the partial derivatives are represented with Derivative, so the pattern needs to match that. Also, I don’t think you want to use CoefficientList as that would accept terms where both your expressions appear. All in all, the following should work:

    In[7]:= (Coefficient[Eq1, #] &) /@ {Derivative[2, 0][U][x1, x2], Derivative[0, 2][V][x1, x2]}
Out[7]= {a, b}

    Here (Coefficient[Eq1, #] &) is an anonymous function that finds the coefficient of the argument, and /@ maps it to the list on the right.

    HTH

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