Allow initial conditions of the form f(0): f(0) in dsolve

[asmeurer]

There was a check that the replacement value does not contain f, but this
makes perfect sense. The check was changed to checking that the value doesn't
contain x.

Fixes #23702

References to other Issues or PRs

Brief description of what is fixed or changed

Other comments

Release Notes

• solvers
  • Allow initial conditions of the form {f(0): f(0)} in dsolve().

[sympy-bot]

✅

Hi, I am the SymPy bot (v167). I'm here to help you write a release notes entry. Please read the guide on how to write release notes.

Your release notes are in good order.

Here is what the release notes will look like:

• solvers
  • Allow initial conditions of the form {f(0): f(0)} in dsolve(). (#23708 by @asmeurer)

This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.11.

Click here to see the pull request description that was parsed.

There was a check that the replacement value does not contain f, but this
makes perfect sense. The check was changed to checking that the value doesn't
contain x.

Fixes #23702

<!-- Your title above should be a short description of what
was changed. Do not include the issue number in the title. -->

#### References to other Issues or PRs
<!-- If this pull request fixes an issue, write "Fixes #NNNN" in that exact
format, e.g. "Fixes #1234" (see
https://tinyurl.com/auto-closing for more information). Also, please
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#### Brief description of what is fixed or changed


#### Other comments


#### Release Notes

<!-- Write the release notes for this release below between the BEGIN and END
statements. The basic format is a bulleted list with the name of the subpackage
and the release note for this PR. For example:

* solvers
  * Added a new solver for logarithmic equations.

* functions
  * Fixed a bug with log of integers.

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<!-- BEGIN RELEASE NOTES -->
- solvers
  - Allow initial conditions of the form `{f(0): f(0)}` in `dsolve()`.
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[oscarbenjamin]

Looks good

[github-actions]

Benchmark results from GitHub Actions

Lower numbers are good, higher numbers are bad. A ratio less than 1
means a speed up and greater than 1 means a slowdown. Green lines
beginning with + are slowdowns (the PR is slower then master or
master is slower than the previous release). Red lines beginning
with - are speedups.

Significantly changed benchmark results (PR vs master)

Significantly changed benchmark results (master vs previous release)

       before           after         ratio
     [77f1d79c]       [89b31eb5]
     <sympy-1.10.1^0>                 
+      96.4±0.4ms        176±0.8ms     1.83  sum.TimeSum.time_doit

Full benchmark results can be found as artifacts in GitHub Actions
(click on checks at the top of the PR).