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I want to generate alphabetical parameters in the differential solution as shown in the figure below: $$\left(\frac{\mathrm d^2}{\mathrm d r^2}+\frac1{r}\frac{\mathrm d}{\mathrm d r}\right)\left(\frac{\mathrm d^2\phi}{\mathrm d r^2}+\frac1{r}\frac{\mathrm d\phi}{\mathrm d r}\right)=0$$

$$\phi=A\ln r+B r^2\ln r+C r^2+D$$ But I get a lot of error warnings when I do this:

L = (D[#, {r, 2}] + 1/r D[#, r]) &;
f[i_] := FromCharacterCode[i + 64]
DSolve[L[(D[φ[r], {r, 2}] + 1/r D[φ[r], r])] == 
  0, φ[r], r, GeneratedParameters -> f]

What can I do to solve this problem better?

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    $\begingroup$ Note that C, D, E, I, etc. are System` symbols, usually with built-in definitions that will cause trouble later. Best to avoid capitals. Perhaps formal capitals?: DSolve[y''[x] == 3 y[x] y'[x] + (3 y[x]^2 + 4 y[x] + 1), y[x], x] /. C[i_] :> FromCharacterCode[ i + 1 + ToCharacterCode["\[FormalCapitalA]"]] $\endgroup$
    – Michael E2
    Commented Apr 25, 2020 at 15:07

2 Answers 2

Reset to default
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Two issues

  1. Define f for Integer.
  2. Return a Symbol.

Technically you can return the string as you are now but you will run into issues down the line.

ClearAll[f]
L = (D[#, {r, 2}] + 1/r D[#, r]) &;
f[i_Integer] := Symbol@FromCharacterCode[i + 64]

Then

DSolve[L[(D[φ[r], {r, 2}] + 1/r D[φ[r], r])] == 0, φ[r], r, GeneratedParameters -> f]

Mathematica graphics

without errors.

Hope this helps.

PS: You may also run into issues with symbols like C and I. This is why it is not recommended to define symbols (outside of packages) that start with capital letters. See here.

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  • $\begingroup$ Thank you very much. In addition, how to generate independent parameters like those in the picture of this problem (for example, only one parameter C)? $\endgroup$
    – A little mouse on the pampas
    Commented Mar 17, 2020 at 2:02
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    $\begingroup$ Try ClearAll[f]; f[i_Integer] := Subscript[\[DoubleStruckF], i]. See the Special Characters guide. $\endgroup$
    – Edmund
    Commented Mar 17, 2020 at 2:23
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Why not replacing afterwards?:

i = 65;
DSolve[L[(D[φ[r], {r, 2}] + 1/r D[φ[r], r])] == 0, φ[r], r] /. 
  C[_] :> FromCharacterCode[i++]

enter image description here

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    $\begingroup$ This approach does not work on DSolve[y''[x] == 3 y[x] y'[x] + (3 y[x]^2 + 4 y[x] + 1), y[x], x]. Better to use DSolve[y''[x] == 3 y[x] y'[x] + (3 y[x]^2 + 4 y[x] + 1), y[x], x] /. C[i_] :> FromCharacterCode[i + 64], though it's better to avoid capitals. $\endgroup$
    – Michael E2
    Commented Apr 25, 2020 at 15:04

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