Fix IsTransitive to require that the group actually acts on the given domain (strictly speaking, the previous behavior was "as documented", but it made no sense and led to bugs elsewhere)
#4907
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... in the case that the group does not act on the given domain
ThomasBreuer
added
kind: bug
... in order to state explicitly that the result of several operations is undefined if the arguments do not define a (transitive) group action (I have also added some tests that document certain situations where one does not get an immediate error message but invalid objects are returned, which then lead to error messages later on. The idea is that it might be necessary to adjust the documentation as soon as some of these tests fail due to code improvements.)
fingolfin
approved these changes
Jun 30, 2022
fingolfin
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the
topic: documentation
fingolfin
changed the title
IsTransitive ...IsTransitive to require that the group actually acts on the given domain
fingolfin
changed the title
IsTransitive to require that the group actually acts on the given domainIsTransitive to require that the group actually acts on the given domain (strictly speaking, the previous behavior was "as documented", but it made no sense and led to bugs elsewhere)
fingolfin
added
release notes: use title
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... in the case that the group does not act on the given domain
Text for release notes
Up to now,
IsTransitive( G, D )did not require thatGacts onD.Thus
truewas returned also whenDwas a proper subset of aG-orbit.From now on,
IsTransitivereturnstrueonly if the given domain is closed under theG-action.The documentation has been changed accordingly.
Additionally, the documentation of several operations (
ActionHomomorphism,Blocks,MaximalBlocks,RepresentativesMinimalBlocks) has been extended by warnings that GAP assumes that the arguments really describe a group action.Further details
The old behaviour of
IsTransitivewas consistent with the old documentation,but other parts of GAP apparently assumed that a group that acts transitively on a domain really acts on this domain.
(For example
RankActionshows an error message that the action must be transitive if the group does not act on the given domain.)There are related situations where GAP does not check whether the group in question really acts on the domain in question, see issue #4904.