integrals: fix free_symbols of transforms with expression variables

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1. maaz7409opened this pull request on 30 Oct 2026description.mdAuthor

   References to other Issues or PRs

   None.

   Brief description of what is fixed or changed

   IntegralTransform.free_symbols put the transform variable itself in the set instead of its free symbols, so for e.g. LaplaceTransform(f(t), t, s - 2) it returned {s - 2} instead of {s}. Because of that:

   • .diff(s) of a transform whose variable is 2*s, s + 1, etc. returned 0 (Laplace, Fourier, Mellin, ...).
   • inverse_laplace_transform(LaplaceTransform(f(t), t, s - 2), s, t) returned 0; it is now left unevaluated.

   The fix uses transform_variable.free_symbols. Added tests for both cases.

   Other comments

   None.

   AI Generation Disclosure

   Claude Code (Claude Opus 5.5) was used to find the bug, to make the edits and commit it. I have reviewed the changes.

   Release Notes

   • integrals
     • Fixed free_symbols of unevaluated integral transforms whose transform variable is an expression. Formerly, LaplaceTransform(f(t), t, 2*s).diff(s) incorrectly gave 0, and inverse_laplace_transform(LaplaceTransform(f(t), t, s - 2), s, t) incorrectly gave 0.
2. sympy-botcommented on 30 Oct 2026

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   Here is what the release notes will look like:

   • integrals
     • Fixed free_symbols of unevaluated integral transforms whose transform variable is an expression. Formerly, LaplaceTransform(f(t), t, 2*s).diff(s) incorrectly gave 0, and inverse_laplace_transform(LaplaceTransform(f(t), t, s - 2), s, t) incorrectly gave 0. (#30895 by @maaz7409)

   This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.15.

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   #### References to other Issues or PRs
   
   
   
   None.
   
   #### Brief description of what is fixed or changed
   
   `IntegralTransform.free_symbols` put the transform variable itself in the set
   instead of its free symbols, so for e.g. `LaplaceTransform(f(t), t, s - 2)`
   it returned `{s - 2}` instead of `{s}`. Because of that:
   
   - `.diff(s)` of a transform whose variable is `2*s`, `s + 1`, etc. returned `0`
     (Laplace, Fourier, Mellin, ...).
   - `inverse_laplace_transform(LaplaceTransform(f(t), t, s - 2), s, t)` returned
     `0`; it is now left unevaluated.
   
   The fix uses `transform_variable.free_symbols`. Added tests for both cases.
   
   #### Other comments
   
   None.
   
   #### AI Generation Disclosure
   
   
   
   Claude Code (Claude Opus 5.5) was used to find the bug, to make the edits and commit it. I have reviewed the changes. 
   
   #### Release Notes
   
   
   
   
   * integrals
     * Fixed `free_symbols` of unevaluated integral transforms whose transform variable is an expression. Formerly, `LaplaceTransform(f(t), t, 2*s).diff(s)` incorrectly gave `0`, and `inverse_laplace_transform(LaplaceTransform(f(t), t, s - 2), s, t)` incorrectly gave `0`.
   

   Update

   The release notes on the wiki have been updated.

3. smichrcommented on 31 Oct 2026Member

   Looks good, thanks!

4. smichrmerged this pull request on 31 Oct 2026

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