# How to make rules for changing parameters

I have one boring problem with changing parameters in printing output. If I have equation

    eq = Plus[Times[a1,Derivative[2,0][q2][x,y]],Times[a2,Derivative[3,1][q1][x,y]]];


I want to replace all derivatives with new variables, for example this

      Derivative[2, 0][q2][x, y]


I want to replace it with $C_{\mathrm{q2}}^{2,0}$ and for all of them on the same way to get eq output automatically in this form

$\mathrm{a1}\,C_{\mathrm{q2}}^{2,0}+\mathrm{a2}\,C_{\mathrm{q1}}^{3,1}$

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– Silvia Jun 18 '13 at 21:18
there is not explanation hoe to change them automatically – George Mills Jun 18 '13 at 21:26
I'm not sure about what you mean by changing parameters. But you might also be interested in RuleDelayed (:>). – Silvia Jun 18 '13 at 21:26

If I understand you correctly, this might be what you're looking for:

eq /. Derivative[n__][q_][__] :>
RawBoxes[
SubsuperscriptBox["C",
ToString[q],
RowBox[Riffle[{n}, ","]]
]]


Please note the using of SubsuperscriptBox and RawBoxes, which will prevent superscript being evaluated as Power.

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