Dear Leonard, Dear Forum,

> I would like to know what the policy is concerning the groups
> in the the very useful GAP libraries of transitive permutation groups and 
> of primitive permutation groups (and any other group library you wish to 
> comment on) 
> as regards to any possible changes wihen changes are made in GAP. In 
> particular, 
> were the actual groups in the library, their GeneratorsOfGroup, and their 
> indexing 
> within the library fixed in perpetuity when the library was initially made 
> available in GAP4,
> and if not, what is the mechanism to alert users to any changes? This is 
> important
> for being able to repeat, check, report on, and extend results made using 
> specific 
> groups in a given  library.

We guarantee the following for the most prominent group libraries. (Guarantees 
are not exactly the same, as the construction processes differ).

- In the library of small groups, the numbering of isomorphism types is 
guaranteed to be stable. Actual group generators, or the PC presentations used 
could theoretically change (though there is no intention to do so, unless it 
would reflect a deeper mathematical understanding, and if this happened I'd 
expect an explicit announcement in the release notes.)

- In the library of transitive groups, the actual permutation groups (but not 
the generators used) are guaranteed to be stable. So you could refer to a 
particular point set and a particular group in a combinatorial construction.

- The *old* library of primitive groups had in fact an unstable numbering, 
dependent on the implementation of the subgroup lattice algorithm. This library 
was therefore retired with the release of GAP 4.2.

- The *new* library of primitive groups, release with GAP 4.2, guarantees a 
stable numbering (which for degree up to 50 is compatible with lists published 
in the literature), but does not guarantee a fixed S_n representative or 
particular generators.

(To the best of my knowledge the numbering of groups is compatible with other 
systems that provide such libraries.)

There is no mechanism for explicitly alerting users of changes in the 
non-guaranteed properties. A change in the guaranteed properties would 
presumably mean that the library was retired and replaced with a new library.

Needless to say, any such promises are only worth as much as the promiser, as 
anyone who endowed a chantry before 1545 can tell you.



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