Mod-p Group Cohomology Package

This is the documentation for our Sage package on the computation of modular cohomology rings of finite groups.

Summary

The source code consists of

  • Python and Cython extension modules as well as Singular and GAP functions written by Simon King,
  • C-programs and Gap functions written by David Green, and
  • a modified version of parts of the Aachen C MeatAxe.

The package comprises a data base of the cohomology rings of all groups of order 64, and can access a repository of the cohomology rings of all groups of order 128, all but 6 groups of order 243, of the Sylow 2-subgroup of the Higman-Sims group, and of the Sylow 2-subgroup of the third Conway group. These data were produced with our package.

Since version 2.0, it can also compute the modular cohomology rings of non prime power groups. In particular, it allows for the computation of the modular cohomology for various primes of the first three Janko groups, of Mathieu groups 11, 12, 22 and 23, of the McLaughlin group, of SuzukiGroup(8), of the Higman-Sims group and of the third Conway group. Here are the computational results. It is planned (but not done yet) to include these cohomology rings in our repository.

The standard way of creating cohomology rings is documented in pGroupCohomology. More details on the available methods can be found in the factory, cohomology and modular_cohomology modules. There are also five other modules used in the background, which may be less interesting to a casual user.

The computation of the modular cohomology rings of non prime power groups is reduced to the case of prime power groups, possibly in several steps, by virtue of the stable element method. The cohomology computation for prime power groups is based on the construction of a minimal free resolution.

In both cases, we follow Jon Carlson’s basic approach to compute an approximation of the cohomology ring in increasing degree, and to use criteria to prove that at some point the approximation is actually isomorphic to the cohomology ring.

We use completeness criteria proposed by Dave Benson, David Green, Simon King and Peter Symonds. The construction of minimal free resolutions is based on an algorithm of David Green.

Installation

Before installing our package, make sure that the Small Groups library of Hans Ulrich Besche, Bettina Eick and Eamonn O’Brien is installed in your copy of Sage. You can install it during a Sage session by

sage: install_package('database_gap')

Till summer 2015, the latest “official” version 2.1.4 of our package could be installed by

sage: install_package('p_group_cohomology')

However, version 2.1.4 is an old-style package, and moreover it can’t install because of backward incompatible changes in Sage. We currently provide a fix against the backward incompatible changes (version 2.1.6) that can be installed on the command line (hence, not in a running Sage session) by

sage -p http://users.minet.uni-jena.de/cohomology/p_group_cohomology-2.1.6.spkg

Version 2.1.6 still is an old-style package and can thus not become an official Sage package. Version 3.0 will be new-style.

Since version 2.1.4, the optimization level has been increased, which actually gives faster code. However, this requires a relatively recent version of gcc. With older compiler versions, installation of this package will fail with a warning hinting to upgrade gcc. On the other hand, gcc version 5.2.1 results in errors when building MeatAxe. Thus, it is best to use gcc 4.9.3.

Since version 2.1, the package works both on little and big endian machines. We are sorry for the long installation time.

Testing

The Cython and Python parts of the package have 100% doctest coverage, but be warned that running the test suite requires a considerable amount of time (easily one hour if a single thread is used). If the environment variable SAGE_CHECK is set to yes, the test script is launched right after installing the package.

The tests are parallelised, provided that Sage version at least 4.6 is used. The number of threads can be bounded using the environment variable SAGE_NUM_THREADS or MAKE (giving the latter a value such as "make -j4".

Documentation

If the environment variable SAGE_SPKG_INSTALL_DOCS is set to yes, then the documentation of our spkg is automatically created and put into SAGE_ROOT/local/share/doc/p_group_cohomology/html/.

Acknowledgements

The development of the initial version of this SPKG was funded by by the German Science Foundation, DFG project GR 1585/4–1, and mainly accomplished at Friedrich Schiller University Jena.

Since version 1.0.1, the further work on this SPKG was funded by Marie Curie grant MTKD-CT-2006-042685 and was pursued at the National University of Ireland, Galway. Since version 2.1.2, the project has returned to Jena and was funded by DFG project KI 861/2–1.

We thank William Stein for giving us access to various computers on which we could build test the SPKG and on which some huge computations could be completed, and acknowledge the support by National Science Foundation Grant No. DMS-0821725.

We are also grateful to William Stein and David Joyner for critical comments and for testing the installation of our package on a large variety of platforms. Suggestions of Mikael Vejdemo Johansson and John Palmieri were very valuable for verifying the code on the computation of Massey products.

We thank Mathieu Dutour Sikiric for his explanations how to keep track of large lists of double cosets in GAP. We are also grateful to the GAP support group for solving various technical problems that became imminent when dealing with non prime power groups.

We thank Peter Symonds for interesting discussions, in particular for suggesting to use the Poincare series in a completeness criterion.

Versions

  • v2.1.6: Cope with several backwards incompatible API changes in SageMath. Nothing has changed in the mathematical part of the code, with respect to v2.1.5.
  • v2.1.5: Cope with several backwards incompatible changes in SageMath. Improved computation of the nil-radical, including a degree-wise computation. Methods is_nilpotent and nilpotency_degree for cohomology ring elements. Various improvements for the computation of depth and filter degree type.
  • v2.1.4: Consequently compute parameters for the complete cohomology ring rather than only for the ring approximation. Better portability by using os.path. Replace SAGE_DATA by SAGE_SHARE. Use urllib2. Protocol output tells which instance’s method is being called.
  • v2.1.3: Improved heuristics to choose between Hilbert-Poincare and Symonds criteria. Cope with Cython’s new “name mangling” for double underscore attributes. Allow storing of “permanent results” that are indexed in terms of data in Gap. In some situations, use a lower bound for the depth, if the actual depth is too difficult to obtain. Switch to a new location for the public web repository, which became necessary by a hardware problem. Fix the creation of symbolic links from a private data base to a public data base of cohomoloy rings. Fix comparison of an MTX matrix with None.
  • v2.1.2: Cope with the new versions of Cython and Singular. Use Sage’s coercion model more properly. Build the documentation locally.
  • v2.1.1: Usage of symbolic links to the public database, so that one can use (but of course not install) this package even without write permission in SAGE_DATA. Restructuring the code. Parallel testing is now only permitted if ticket #10004 is applied (September 2010).
  • v2.1: Support for big endian machines. 100% doctest coverage, parallel testing. New: Essential and depth essential ideals. Improved completion tests. (September 2010).
  • v2.0: Cohomology of non prime power groups (April 2010).
  • v1.2.p0: Improved test for the presence of the Small Groups library (thanks to Dima Pasechnik, March 2010).
  • v1.2: Modified printing for cocycles; minor bug fixes and code improvements. New: Persistent Group Cohomology (bar codes), based on ideas of Graham Ellis and Simon King (October 2009).
  • v1.1: Added restricted Massey powers and general Massey products (August 2009).
  • v1.0.2: Fixes some bugs (July 2009).
  • v1.0.1: First public version in GPL 2 or later (July 2009)

Licence

This document and our data bases of cohomology rings are licensed under a Creative Commons Attribution-Share Alike 3.0 License.

The code of our package is licensed under the GNU General Public License (GPL) version 2 or later, at your choice.

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