#26750 closed enhancement (fixed)
Use GroupHomset_libgap for permutation groups, as well
Reported by:  soehms  Owned by:  

Priority:  major  Milestone:  sage8.6 
Component:  group theory  Keywords:  permutation group, homomorphism, libgap 
Cc:  tscrim, sbrandhorst  Merged in:  
Authors:  Sebastian Oehms  Reviewers:  Simon Brandhorst 
Report Upstream:  N/A  Work issues:  
Branch:  66ceb94 (Commits, GitHub, GitLab)  Commit:  
Dependencies:  #26420  Stopgaps: 
Description
At the moment the construction of group homomorphisms between groups implemented under the classes ParentLibGAP
and PermutationGroup_generic
(and vice versa) is not possible in sage using the method hom
. For example, it isn't possible to construct the natural projection from symplectic groups onto the corresponding projective group although this is possible using GAP:
sage: Sp43 = Sp(4,3) sage: PSp43 = PSp(4,3) sage: natProj = Sp43.hom(PSp43.gens()) Traceback (most recent call last): ... TypeError: unable to convert [(3,4)(6,7)(9,10)(12,13)(17,20)(18,21)(19,22)(23,32)(24,33)(25,34)(26,38)(27,39)(28,40)(29,35)(30,36)(31,37), (1,5,14,17,27,22,19,36,3)(2,6,32)(4,7,23,20,37,13,16,26,40)(8,24,29,30,39,10,33,11,34)(9,15,35)(12,25,38)(21,28,31)] to an element of Set of Morphisms from Symplectic Group of degree 4 over Finite Field of size 3 to The projective symplectic linear group of degree 4 over Finite Field of size 3 in Category of finite groups
The reason for this is that the constructor of the class GroupHomset_libgap
explicitly checks both groups to be instances of ParentLibGAP
. So an easy way to have hom
work in such cases, as well, would be to allow PermutationGroup_generic
additionally.
An alternative option is to shift PermutationGroup_generic
into the ParentLibGAP
framework. But this would cause a lot of work to have all doctests pass through. The main problem here is that the element class of PermutationGroup_generic still has parent
as second (optional) argument (in opposite to ParentLibGAP
).
Therefore, this ticket will follow the first option!
Change History (37)
comment:1 Changed 3 years ago by
 Branch set to u/soehms/libgap_morphism_permgroup26750
comment:2 Changed 3 years ago by
 Commit set to 28f39df7a269aad93c918b7039a7da68b1869ded
 Dependencies set to #26420
 Status changed from new to needs_review
comment:3 Changed 3 years ago by
I am not convinced the function _obtain_gap_hom
is useful/necessary.
It seems to be used only for the natural_map
method. But then the code just belongs there.
But indeed one could replace
gens = [g.gap() for g in dom.gens()]
by
gens = dom.gap().GeneratorsOfGroup()
in the _element_constructor_
and likewise for the images in natural map. Would that fix the overflow?
comment:4 Changed 3 years ago by
 Commit changed from 28f39df7a269aad93c918b7039a7da68b1869ded to deaa64722319dc847871b77b15cab2efbb972850
Branch pushed to git repo; I updated commit sha1. New commits:
deaa647  remove unnecessary method _obtain_gap_hom

comment:5 Changed 3 years ago by
Thanks for looking at this, Simon! Of course you are right: This method has been a stupid idea! I corrected as you suggested.
comment:6 Changed 3 years ago by
 Commit changed from deaa64722319dc847871b77b15cab2efbb972850 to 2b74ce465c362921430ba0b58b7c5e207ad3241d
Branch pushed to git repo; I updated commit sha1. New commits:
0133335  implement subgroup / ambient methods for MatrixGroup_base

747bc16  removed unused keyword

c3a92a4  Merge branch 'develop' into matrix_groups_subgroups25894

901441b  Merge branch 'develop' into matrix_groups_subgroups25894

b02eb31  Merge branch 'libgap_morphism_permgroup26750' into cubic_braid_group

be4674b  initial implementation

6a07854  Merge branch 'develop' into adapt_unitary_burau26657

806d0c6  Merge branch 'adapt_unitary_burau26657' into cubic_braid_group

464b784  Merge branch 'libgap_morphism_permgroup26750' into cubic_braid_group

2b74ce4  restore deleted doctest

comment:7 Changed 3 years ago by
New commits:
0133335  implement subgroup / ambient methods for MatrixGroup_base

747bc16  removed unused keyword

c3a92a4  Merge branch 'develop' into matrix_groups_subgroups25894

901441b  Merge branch 'develop' into matrix_groups_subgroups25894

b02eb31  Merge branch 'libgap_morphism_permgroup26750' into cubic_braid_group

be4674b  initial implementation

6a07854  Merge branch 'develop' into adapt_unitary_burau26657

806d0c6  Merge branch 'adapt_unitary_burau26657' into cubic_braid_group

464b784  Merge branch 'libgap_morphism_permgroup26750' into cubic_braid_group

2b74ce4  restore deleted doctest

comment:8 Changed 3 years ago by
New commits:
0133335  implement subgroup / ambient methods for MatrixGroup_base

747bc16  removed unused keyword

c3a92a4  Merge branch 'develop' into matrix_groups_subgroups25894

901441b  Merge branch 'develop' into matrix_groups_subgroups25894

b02eb31  Merge branch 'libgap_morphism_permgroup26750' into cubic_braid_group

be4674b  initial implementation

6a07854  Merge branch 'develop' into adapt_unitary_burau26657

806d0c6  Merge branch 'adapt_unitary_burau26657' into cubic_braid_group

464b784  Merge branch 'libgap_morphism_permgroup26750' into cubic_braid_group

2b74ce4  restore deleted doctest

comment:9 Changed 3 years ago by
I think I need some help. I wanted to restore the doctest of _obtain_gap_hom
in _element_constructor_
but pushed from the wrong branch.
How can I go back to commit deaa64722319dc847871b77b15cab2efbb972850?
comment:10 Changed 3 years ago by
 Commit changed from 2b74ce465c362921430ba0b58b7c5e207ad3241d to 68582de9123703c95fcc065e56599da12e87ec9d
Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:
68582de  restore deleted doctest

comment:11 Changed 3 years ago by
I think I got it!
comment:12 Changed 3 years ago by
in the _element_constructor
method:
if list(x) == codom.gens(): # avoid buffer size overflow in the interface imgs = codom.gap().GeneratorsOfGroup()
this seems to fit better with the natural_map
method? Put it there.
comment:13 Changed 3 years ago by
 Commit changed from 68582de9123703c95fcc065e56599da12e87ec9d to a119816b247bfcfe18330326067e2f908b342b04
Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:
a119816  revert changes in _element_constructor_ and improve method gap of permutation groups

comment:14 Changed 3 years ago by
Thanks for your useful hints, Simon! There where two problems:
1) I provided the wrong example, so that you did not have a chance to see what these lines of code should be good for. The correct example needs to have an instance of PermutationGroup_subgroup
as codomain and be large enough to produce:
python2: libgap.c:184: libgap_get_input: Assertion `strlen(libGAP_stdin_buffer) < length' failed
2) The origin for this error, has not been in the _element_constructor_
method. So any changes here where useless, as you pointed out. Actually, the problem was that GAP didn't know about the subgroup construction.
So I revert all changes of code with respect to the _element_constructor_
and provide the example which caused the trouble in its docstring. I inserted a line of code in the constructor of the PermutationGroup_subgroup
class to have the gap
method of such a subgroup work well. This line I explain by a doctest in gap
.
In addition I change the implementation of gap
, since I realized that the former one can cause pickling errors.
comment:15 Changed 3 years ago by
 Branch u/soehms/libgap_morphism_permgroup26750 deleted
 Commit a119816b247bfcfe18330326067e2f908b342b04 deleted
comment:16 Changed 3 years ago by
 Branch set to u/soehms/libgap_morphism_permgroup_26750
comment:17 followup: ↓ 18 Changed 3 years ago by
 Commit set to e1e4da03582f9674e1628a02e6b502d9dfdcee85
It seems you are caching twice here. Once with the decorator once inside the method.
@cached_method def gap(self):
New commits:
e1e4da0  resetting to a new branch because of merge error

comment:18 in reply to: ↑ 17 ; followup: ↓ 21 Changed 3 years ago by
Replying to sbrandhorst:
It seems you are caching twice here. Once with the decorator once inside the method.
@cached_method def gap(self):
New commits:
e1e4da0 resetting to a new branch because of merge error
I inserted the decorator since I removed the explixit caching via the _libgap
attribute (since this caused pickling problems). I am not aware that there is an implicite caching mechanism via the element construction of class Gap
. Can you explain how this is working?
comment:19 Changed 3 years ago by
I've linked this ticket to #26902, which aims at planning and tracking conversion of Sage library code from pexpect gap to libgap.
comment:20 Changed 3 years ago by
Could you take care of pyflakes' advice (see patchbot output) and remove unneeded imports:
========== pyflakes ========== git checkout patchbot/ticket_merged src/sage/categories/homset.py:73: 'sage.misc.constant_function.ConstantFunction' imported but unused src/sage/categories/homset.py:75: 'types' imported but unused found 2 pyflakes errors in the modified files
comment:21 in reply to: ↑ 18 ; followup: ↓ 22 Changed 3 years ago by
Replying to soehms:
I inserted the decorator since I removed the explixit caching via the
_libgap
attribute (since this caused pickling problems). I am not aware that there is an implicite caching mechanism via the element construction of classGap
. Can you explain how this is working?
It still seems to be there:
+ @cached_method + def gap(self): + if self._libgap is not None: + return self._libgap + from sage.libs.gap.libgap import libgap + return libgap(self)
To me it looks like there are 2 caches once via the _libgap attribute and once in the @cached method
Say if one first calls .gap()
and then the _libgap attribute is set somewhere
comment:22 in reply to: ↑ 21 Changed 3 years ago by
Replying to sbrandhorst:
Replying to soehms: To me it looks like there are 2 caches once via the _libgap attribute and once in the @cached method Say if one first calls
.gap()
and then the _libgap attribute is set somewhere
In general it isn't cached via that attribute (I've removed that in my last change):
sage: S = SymmetricGroup(6) sage: print S._libgap None sage: S.gap() Sym( [ 1 .. 6 ] ) sage: print S._libgap None
comment:23 Changed 3 years ago by
 Commit changed from e1e4da03582f9674e1628a02e6b502d9dfdcee85 to 5b2c3816259709db4e033bef1c3ca0ff14abfd21
comment:24 Changed 3 years ago by
+ from sage.libs.gap.element import GapElement_Permutation + if isinstance(img_gap, GapElement_Permutation): + return self.codomain()(img_gap.sage())
What happens here is a conversion of a gap element to an element of a permutation group.
It would be more useful to let this be handled by the permuation group code
i.e. in the element constructor of permutation groups and not in the _call_
method of the morphism.
Also there are a few little python style remarks for the doctests, like
+ sage: GS=G.subgroup([G.gen(0)])
should be
+ sage: GS = G.subgroup([G.gen(0)])
comment:25 Changed 3 years ago by
 Branch changed from u/soehms/libgap_morphism_permgroup_26750 to u/sbrandhorst/libgap_morphism_permgroup_26750
comment:26 Changed 3 years ago by
 Branch changed from u/sbrandhorst/libgap_morphism_permgroup_26750 to u/soehms/libgap_morphism_permgroup_26750
comment:27 Changed 3 years ago by
 Commit changed from 5b2c3816259709db4e033bef1c3ca0ff14abfd21 to 09b469601aa8cad0fa1f51baf225a8fb807eaa0a
I follow your suggestion concerning the _call_
method and I revert one of your style changes because of a failing doctest (see patchbot).
New commits:
54a7f1b  Merge branch 'develop' into t/26750/libgap_morphism_permgroup_26750

8bd5eff  cosmetic changes to the docs

09b4696  move conversion to element construction of permutation

comment:28 Changed 3 years ago by
 Branch changed from u/soehms/libgap_morphism_permgroup_26750 to u/sbrandhorst/libgap_morphism_permgroup_26750
comment:29 followup: ↓ 32 Changed 3 years ago by
 Commit changed from 09b469601aa8cad0fa1f51baf225a8fb807eaa0a to e0feb001f33e1f78efd180d38a6ddbbe6741ebeb
Positive review if you are happy with my changes & we have a green bot
New commits:
e0feb00  changes to docs and the permutation group and subgroup constructor

comment:30 followup: ↓ 33 Changed 3 years ago by
btw. if you split up your tickets into smaller ones, they are easier to review
comment:31 Changed 3 years ago by
 Branch changed from u/sbrandhorst/libgap_morphism_permgroup_26750 to u/soehms/libgap_morphism_permgroup_26750
comment:32 in reply to: ↑ 29 Changed 3 years ago by
 Commit changed from e0feb001f33e1f78efd180d38a6ddbbe6741ebeb to 66ceb94036041b2bf05e1f4abf91ea8bdeac862a
 Reviewers set to Simon Brandhorst
Replying to sbrandhorst:
Positive review if you are happy with my changes & we have a green bot
New commits:
e0feb00 changes to docs and the permutation group and subgroup constructor
This is a good improvement in order to get the permutation groups closer to libgap!
I just do two docstring adaptions and rename your GapElement
since that name is already used globally with respect to sage.interfaces.gap
Thanks for the review, Simon. I wish you a merry Christmas!
New commits:
66ceb94  adapt docstrings to former change and ticket #26889

comment:33 in reply to: ↑ 30 Changed 3 years ago by
Replying to sbrandhorst:
btw. if you split up your tickets into smaller ones, they are easier to review
Sure! I will try to take care of that the next time!
comment:34 Changed 3 years ago by
 Status changed from needs_review to positive_review
comment:35 Changed 3 years ago by
 Branch changed from u/soehms/libgap_morphism_permgroup_26750 to 66ceb94036041b2bf05e1f4abf91ea8bdeac862a
 Resolution set to fixed
 Status changed from positive_review to closed
comment:36 Changed 3 years ago by
 Commit 66ceb94036041b2bf05e1f4abf91ea8bdeac862a deleted
 Milestone changed from sage8.5 to sage8.6
comment:37 Changed 3 years ago by
I've created a follow up ticket #27234 since by the improvements in connection with GAP 4.10 upgrade, the implementation of the gap
method of class PermutationGroupElement
can be done properly, right now.
I do the following:
1) I add the method
gap
to the classesPermutationGroup_generic
andPermutationGroupElement
to achieve compatibility with classGroupHomset_libgap
and its elements class. For the same reason I add the method_subgroup_constructor
toPermutationGroup_generic
(needed in the methodspushforward
,kernel
andpreimage
inGroupMorphism_libgap
)2) I implement the method
_Hom_
inPermutationGroup_generic
to havehom
use classGroupHomset_libgap
.3) In the module
libgap_morphism
I addPermutationGroup_generic
to the list of allowed parentinstances at all relevant pieces of code. Furthermore, I changed the_element_constructor_
in order to avoid buffer size overflow by transferring long lists of large permutation generators to GAP. This is done in a new local method_obtain_gap_hom
.4) I take the occasion to implement two methods from framework:
section
of classMap
andnatural_map
of classHomsetWithBase
improving their default implementation using the possibilities ofGroupHomset_libgap
and its element class.New commits:
group morphism constructor from a libgap morphism
a doctest
docs
correct docstring for argument `gap_hom`
Merge commit '21cb66449cd424f94d15bd695fa986a0ce40c30c' of git://trac.sagemath.org/sage into libgap_morphism_permgroup26750
initial version