Added proper_divisor and proper_divisor_count functions

[czgdp1807]

References to other Issues or PRs

Fixes #15927
#15928

Brief description of what is fixed or changed

Other comments

Release Notes

• ntheory
  • Added proper_divisor and proper_divisor_count functions

[sympy-bot]

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• ntheory
  • Added proper_divisor and proper_divisor_count functions (#18085 by @adhoc-king and @czgdp1807)

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<!-- 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
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Fixes #15927
https://github.com/sympy/sympy/pull/15928

#### Brief description of what is fixed or changed


#### Other comments


#### Release Notes

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<!-- BEGIN RELEASE NOTES -->
- ntheory
    - Added proper_divisor and proper_divisor_count functions
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Update

The release notes on the wiki have been updated.

[sylee957]

Sphinx build is failing
Add these lines in ntheory.rst

.. autofunction:: proper_divisors

.. autofunction:: proper_divisor_count

[czgdp1807]

Restarted the builds due to irrelevant failures in previous one.

[codecov]

Codecov Report

Merging #18085 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##            master    #18085      +/-   ##
============================================
+ Coverage   74.884%   74.934%   +0.05%     
============================================
  Files          641       642       +1     
  Lines       166564    166965     +401     
  Branches     39179     39281     +102     
============================================
+ Hits        124730    125114     +384     
- Misses       36317     36318       +1     
- Partials      5517      5533      +16

[czgdp1807]

@jksuom Is it ready to be merged? Please take a look.

[jksuom]

Looks good to me.