Computer checking of the subgroup data

Abstract

Most of the data in Part 2 of this volume were checked by a computer-algebra program based upon the package Cryst, which in turn is an extension of the computer-algebra program GAP. The basic capabilities of Cryst are described. A space group is represented as a group of augmented matrices. The implementation of a membership test for such groups is explained, along with how the test can be used to determine whether two space groups are equal or whether one is a subgroup of the other. The algorithm for computing the maximal subgroups of space groups is also outlined. The tests that were performed are then described. The program parsed the LaTeX source files that were used to produce the tables of data. All the data tabulated for the low-index subgroups were checked for correctness and consistency. The low-index subgroups were recomputed from scratch and compared with the tabulated subgroups. Unfortunately, such tests could not be applied to the infinite series of maximal isomorphic subgroups of arbitrarily high index. This chapter is also available as HTML from the International Tables Online site hosted by the IUCr.