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I wonder why I can not have a correct integration form on Mathematica with some indexed variables.

Indeed, when I write :

Integrate[r/Subscript[r, L], {r, 0, Subscript[r, L]}]

I have the following result :

enter image description here

However, when I write :

Integrate[r/Subscript[q, L], {r, 0, Subscript[q, L]}]

I obtain the expected (computed) expression :

enter image description here

Could you explain me where is my error ? and how can I have the expected result with first expression ?

Best,

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  • $\begingroup$ @DanielHuber That's not quite the case, though close. In[79]:= Integrate[q*Subscript[r, n], {Subscript[r, n], 0, q}] Out[79]= q^3/2. The issue with the one in this post is that r is the integration variable, and the integrand contains Subscript[r, L] which is viewed as an unknown function of r. So Integrate is flummoxed. $\endgroup$
    – Daniel Lichtblau
    Commented May 17, 2023 at 20:07
  • $\begingroup$ Ok, thanks for this remark. But is it possible to prevent the problem ? I would like to use rL as a variable in order to produce more clear formula, rather than using another variable name. To summarize, is-it possible to constrain Mathematica to consider rL not as an unknown function but as a parameter independant from r ? $\endgroup$
    – Nathan Gege
    Commented May 17, 2023 at 20:10

1 Answer 1

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One idea is to create an environment where subscripts are rendered inert, and then the expression is evaluated. For example:

SetAttributes[InactivatedSubscripts,HoldAll];

InactivatedSubscripts[expr_]:=Block[{CompressedData},
    With[{h=Hold[expr]/.s_Subscript:>RuleCondition@CompressedData[Compress@s]},
        ReleaseHold[h]
    ]
]

Then:

InactivatedSubscripts[
    Integrate[r/Subscript[r,L],{r,0,Subscript[r,L]}]
]

Subscript[r, L]/2

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