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This file describes changes in the GAP package 'polycyclic'.

2.18 (2026-04-09)

  • Let Centralizer( U, g ) work if g is not contained in U
  • Improve Intersection to work in more cases (instead of erroring out), specifically if one of the arguments is finite
  • Various janitorial changes

2.17 (2025-08-28)

  • Add a method SemidirectProduct( N, alpha, G) where N and G are both PcpGroups (contributed by Sam Tertooy)
  • Enhance AbelianPcpGroup and AbelianGroupCons to support infinity in the list of orders (implemented by Sam Tertooy)
  • Enhance Centralizer to fall back on generic GAP methods, so it can be computed in more cases
  • Fix correctness bugs, where wrong results could be returned, in the following functions:
    • AddToIgs
    • Centralizer could sometimes set an incorrect igs in the computed centralizer, which could lead to incorrect results later on
    • ConjugacyElementsBySeries
    • FrattiniSubgroup for finite pcp groups (reported by Heiko Dietrich)
    • IsConjugate (reported and fixed by Sam Tertooy)
    • IsNormal sometimes returned true even if the inputs did not normalise each other
    • OrbitIntegralAction
    • Subgroup sometimes produced a group with invalid Pcp and e.g. membership tests could fail
  • Fixed a bug in AbelianGroup resp. AbelianGroupCons method for IsPcpGroup and also in AbelianPcpGroup, DihedralPcpGroup, which resulted in either an unexpected break loop or in a corrupted group when used to create a group with some but not all generators of order 1
  • Fix further bugs where an error was raised unexpectedly:
    • ConjugacyClasses sand ConjugacyClassesSubgroups sometimes did run into errors or took far too long
    • IsSingleValued (and possibly CoKernelOfMultiplicativeGeneralMapping) for certain trivial maps
    • Random for trivial groups (reported and fixed by Sam Tertooy)
    • PreImagesSet if the input set is not contained in the image of the homomorphism
  • Fix SchurCovers to always return list, even if only one cover is found (fix by Sam Tertooy)
  • Support and use NC versions of PreImages, PreImagesElm, PreImagesSet and PreImagesRepresetnative (contributed by Chris Wensley)
  • Made most global functions and variables read-only to catch code accidentally overriding them
  • Removed some unused code
  • Various janitorial changes

2.16 (2020-07-25)

  • Fixed a bug in NormalIntersection which could lead to wrong results; this also affected other operations, such Core, Intersection
  • Fixed PreImagesRepresentative for trivial homomorphisms (it used to return the identity fo the source as preimage for all elements in the range, instead of returning fail for all but the identity of the range)
  • Fixed some bugs in AddToIgs and AddTailInfo
  • Some janitorial changes

2.15.1 (2019-10-03)

  • Fixed a regression that could lead to an infinite loop in IsomorphismPcGroup

2.15 (2019-09-27)

  • Added license information to package metadata
  • Added support for random sources to Random method for pcp-groups
  • Documented IsPcpGroup and IsPcpElementCollection
  • Increased rank for IsomorphismPcGroup and IsomorphismFpGroup methods for pcp-groups, to ensure they are still used when all GAP packages are loaded
  • Some janitorial changes

2.14 (2018-05-12)

  • Fixed a bug in OneCoboundariesCR which lead to an error in OneCohomologyCR
  • Fixed a bug where the normal closure of an abelian subgroup could end up being flagged as abelian, even though it was not
  • Restored compatibility with GAP versions before 4.9

2.13.1 (2018-04-27)

  • Removed a regression test case which failed if no other packages are loaded

2.13 (2018-04-26)

  • Fixed bug in IsConjugate
  • Fixed building the manual via makedoc.g on case-sensitive file systems
  • Replaced immediate methods for IsTorsionFree and IsFreeAbelian by implications, which have zero overhead, while immediate methods can slow down GAP
  • Improved performance of UnitriangularPcpGroup for large n

2.12 (2018-03-18)

  • Improved performance of some orbit algorithms by using dictionaries
  • Improved performance of AddToIgs for some examples where it previously performed very badly
  • Added custom IsSingleValued method for group homomorphisms whose Source is an polycyclic groups, which can avoid an endless loop when the range is an infinite group
  • Fixed bug in NormalizerPcpGroup which could result in a break loop
  • Fixed bug in ComplementClassesCR which could result in a break loop
  • Fixed bug in OrbitIntegralAction which could result in a break loop
  • Fixed bug in StabilizerIntegralAction which could result in a break loop
  • Fixed bug in AddToIgs for infinite groups which could result in an invalid output leading to strange results
  • Fixed IsConjugate for pcp group elements to always return true and false (instead of an element which conjugates the inputs to each other)
  • Corrected documentation for HeisenbergPcpGroup to give correct number of generators, an correct Hirsch length
  • Corrected and clarified InfiniteMetacyclicPcpGroup documentation
  • Deprecated NaturalHomomorphism, use NaturalHomomorphismByNormalSubgroup instead (which is a standard GAP operation)
  • Removed left-over traces of Schur towers in the manual and elsewhere
  • Added more tests cases
  • Changed tests to using TestDirectory
  • Various minor tweaks

2.11 (2013-03-07)

  • Added a fast SylowSubgroup method (via IsomorphismPcGroup)
  • Add FreeAbelianGroup constructor method (feature will only be available in a future GAP release)
  • Replaced some internal code dealing with integer matrices with calls to equivalent GAP functions; for some things (e.g. inverting a matrix), this can be a lot faster
  • Fixed regressions in 2-cohomology code (introduced in 2.9), which caused TwoCoboundariesCR and TwoCohomologyCR to produce errors or wrong results
  • Fixed infinite recursion in LowerCentralSeriesOfGroup for non-nilpotent pcp groups (thanks to Andreas Distler for noticing and fixing this)
  • Removed support for GAP 4.4, now GAP 4.5 or newer is required
  • Removed some obsolete code
  • Removed or hid multiple undocumented internal functions (such as AsMat, IntMat, OnVectorspace, VERIFY, ...) to reduce the pollution of the global namespace
  • Various minor tweaks

2.10.1 (2012-06-01)

  • Fixed generic IsFreeAbelian method to only apply to finitely generated groups
  • Removed "name strings" from two InstallImmediateMethod calls; this should have no effect on any user, and is done to silence some pedantic warnings in the GAP test suite

2.10 (2012-05-31)

  • Added methods for GAP's Epicentre and EpimorphismSchurCover attribute
  • Added group constructors that allow construction extraspecial groups as well as alternating and symmetric groups of degree <= 4 as pcp groups
  • Changed SchurExtension and SchurExtensionEpimorphism into attributes
  • Changed IsomorphismPcpGroup for pcp groups, now returns identity map
  • Changed SchurCovering to be a synonym for GAP's SchurCover attribute
  • Fixed regression in AddFieldCR which caused incorrect errors (e.g. when testing whether a pcp group is torsion free)
  • Fixed some warnings by adding a IsGeneratorsOfMagmaWithInverses method for pcp element collections
  • Fixed several bugs resulting in errors when computing Schur extensions, nonabelian exterior and tensor squares, and so on, but only if the argument was a subgroup of a pcp group
  • Fixed computing Schur extensions, nonabelian exterior and tensor squares etc. of the infinite cyclic group
  • Fixed bug in direct products of pcp groups that could result in wrong embedding and projection maps
  • Fixed error triggered when calling NormalizerOp on two groups that have differing Parent() groups, yet still are subgroups of a common overgroup
  • Removed some dead obsolete code

2.9 (2012-01-12)

  • Updated README
  • Added GPL license text
  • Added this CHANGES file
  • Added Max Horn to authors / maintainer list
  • Removed Werner Nickel from maintainer list
  • Removed compatibility with GAP versions before 4.4
  • Removed redundant IsomorphismPermGroup method
  • Removed redundant IsPcpGHBI group mapping representation
  • Added various group constructors (TrivialGroupCons etc.), so that it is now possible to construct Pcp groups with e.g. TrivialGroup(IsPcpGroup) or DihedralGroup(IsPcpGroup, infinity). Specifically, this works now for cyclic, (elementary) abelian, dihedral, and quaternion groups
  • Added implementations of IndependentGeneratorsOfAbelianGroup and IndependentGeneratorExponents for pcp groups
  • Improved handling of homomorphisms between pcp groups and non-pcp groups
  • Improved validation of input for various functions / methods
  • Improved AbelianPcpGroup to flag the constructed group as abelian
  • Fixed AbelianInvariants to return values that match what the GAP documentation promises the user
  • Fixed a bug that caused TwoCohomologyCR and many related operations to error out if the cohomology record was obtained using CRRecordBySubgroup or CRRecordByPcp
  • Fixed bug comparing homomorphisms between pcp groups by removing the (incorrect) method for this; the default method provided by GAP is now used and returns correct results
  • Fixed ClosureGroup method to not make invalid assumptions about a group's Parent (and thus no longer return incorrect results)
  • Fixed bug causing general mappings from/to pcp groups to be always marked as total, even if they were in fact not
  • Added IsNilpotentByFinite methods for finite and nilpotent groups
  • Added immediate IsTorsionFree method for finite groups
  • Added IsFreeAbelian method for arbitrary groups, turned it into a property
  • Converted documentation to GAPDoc format
  • Replaced internal function DepthOfVec by GAP's PositionNonZero
  • Added (trivial) IsomorphismPcpGroup method for pcp groups
  • Added a String method for pcp elements

2.8.1 (2011-05-24)

  • Use Calcreps2 instead of calcreps2 for compatibility with GAP 4.5
  • Updated homepage URLs

2.8 (2011-01-26)

  • Improved and corrected parts of the manual
  • Removed IsomorphismPcpGroup method for fpgroups, and instead provide and document it as a regular function under the name IsomorphismPcpGroupFromFpGroupWithPcPres
  • Use "-u" option when creating the HTML manual to produce unicode output
  • Removed SchurMultiplicator method and instead install a method for AbelianInvariantsMultiplier

2.7: Never released

2.6 (2009-02-18)

  • Disabled (and removed any mention from the documentation) some code dealing with Schur towers of p-groups of fixed coclass.
  • Fixed email address of Bettina Eick

2.5 (2008-11-25)

  • Added SchurCovers
  • Added dependency on autpgrp package
  • Added GroupHomomorphismByImagesNC implementation for when the source is a Pcp group, but the range is possibly not.
  • Compute size of newly constructed group in PcpGroupByCollectorNC
  • Various other fixes and improvements

2.4 (2008-11-12)

  • Fixed a bug in DirectProduct for PcpGroups

2.3 (2008-11-09)

  • Removed compatibility with GAP versions before 4.3
  • Added WhiteheadQuadraticFunctor
  • Added IsPolycyclicPresentation
  • Added IsomorphismPermGroup
  • Renamed DepthVector -> DepthOfVec
  • Renamed PrintFullPresentation -> PrintPcpPresentation
  • Renamed Tail -> TailOfElm
  • Replaced many uses of BindGlobal by InstallGlobalFunction
  • Improved group homomorphism code
  • Implemented Embedding and Projection for DirectProduct
  • Implemented Embedding for WreathProduct
  • Extended AbelianPcpGroup to accept types of arguments (undocumented)
  • Various other fixes and improvements

2.2 (2007-06-22)

  • Added support for non-abelian tensor and exterior squares
  • TODO: Schur extensions code was also touched?
  • Various other fixes and improvements

2.1 (2006-11-07)

  • Declare in PackageInfo.g that nq is a suggested (but not required) external package, and try harder to work when nq is missing.
  • Rewrote SchurExtension
  • Changed IsomorphismPcGroup to first convert the group to a refined pcp group; also, the resulting homomorphism is now marked as being a group homomorphism.
  • Several existing functions now are "properly" installed via InstallMethod or InstallGlobalFunction
  • Various other fixes and improvements

2.0 (2006-10-23)

1.1 (2003-10-15)

1.0 (???)