Homomorphism shows error for relator element not in generator

[divyanshu132]

Brief description of what is fixed or changed

Previously:

>>> G = PermutationGroup([Permutation(1,2,3,4), Permutation(1,3)])
>>> a = Permutation(1,2,3,4)
>>> homomorphism(G, G, [a], [a])
Traceback (most recent call last): File "<string>", line 1, in <module> File "/base/data/home/apps/s~sympy-live-hrd/56.412575950366508090/sympy/sympy/combinatorics/homomorphisms.py", line 304, in homomorphism if check and not _check_homomorphism(domain, codomain, images): File "/base/data/home/apps/s~sympy-live-hrd/56.412575950366508090/sympy/sympy/combinatorics/homomorphisms.py", line 363, in _check_homomorphism s = _image(r).is_identity File "/base/data/home/apps/s~sympy-live-hrd/56.412575950366508090/sympy/sympy/combinatorics/homomorphisms.py", line 335, in _image s = domain.generators[gens.index(r[i])] 
ValueError: tuple.index(x): x not in tuple

Now:

>>> G = PermutationGroup([Permutation(1,2,3,4), Permutation(1,3)])
>>> a = Permutation(1,2,3,4)
>>> homomorphism(G, G, [a], [a])
raise ValueError("The given images do not define a homomorphism")
ValueError: The given images do not define a homomorphism

Other comments

Release Notes

• combinatorics
  • fix check_homomorphism for elements in relator not in generator

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• combinatorics
  • fix check_homomorphism for elements in relator not in generator (#16394 by @divyanshu132)

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#### Brief description of what is fixed or changed
Previously:
```
>>> G = PermutationGroup([Permutation(1,2,3,4), Permutation(1,3)])
>>> a = Permutation(1,2,3,4)
>>> homomorphism(G, G, [a], [a])
Traceback (most recent call last): File "<string>", line 1, in <module> File "/base/data/home/apps/s~sympy-live-hrd/56.412575950366508090/sympy/sympy/combinatorics/homomorphisms.py", line 304, in homomorphism if check and not _check_homomorphism(domain, codomain, images): File "/base/data/home/apps/s~sympy-live-hrd/56.412575950366508090/sympy/sympy/combinatorics/homomorphisms.py", line 363, in _check_homomorphism s = _image(r).is_identity File "/base/data/home/apps/s~sympy-live-hrd/56.412575950366508090/sympy/sympy/combinatorics/homomorphisms.py", line 335, in _image s = domain.generators[gens.index(r[i])] 
ValueError: tuple.index(x): x not in tuple
```

Now:
```
>>> G = PermutationGroup([Permutation(1,2,3,4), Permutation(1,3)])
>>> a = Permutation(1,2,3,4)
>>> homomorphism(G, G, [a], [a])
raise ValueError("The given images do not define a homomorphism")
ValueError: The given images do not define a homomorphism

```

#### Other comments


#### Release Notes

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https://github.com/sympy/sympy/wiki/Writing-Release-Notes for more information
on how to write release notes. The bot will check your release notes
automatically to see if they are formatted correctly. -->

<!-- BEGIN RELEASE NOTES -->
*  combinatorics
   *  fix check_homomorphism for elements in relator not in generator
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[divyanshu132]

@jksuom please have a look!!

[codecov]

Codecov Report

Merging #16394 into master will increase coverage by 0.051%.
The diff coverage is 50%.

@@              Coverage Diff              @@
##            master    #16394       +/-   ##
=============================================
+ Coverage   73.571%   73.623%   +0.051%     
=============================================
  Files          618       618               
  Lines       158471    158472        +1     
  Branches     37183     37184        +1     
=============================================
+ Hits        116590    116673       +83     
+ Misses       36447     36366       -81     
+ Partials      5434      5433        -1

[divyanshu132]

@jksuom please review this as well.

[oscarbenjamin]

This needs a review. I don't know the combinatorics module well enough though...

[jksuom]

This looks good. Sorry for missing this. Thanks for the reminder, Oscar.

[divyanshu132]

Ohh.. I also forgot this once GSoC started! Thanks @oscarbenjamin!!