Introduction The purpose of this package is to provide functionalities to work with torsion units in (integral) group rings. It implements a method that was developed by I.S. Luthar and I.B.S. Passi in and which was extended by M. Hertweck in . These names also constitute the name of the method, suggested by A. Konovalov: Hertweck-Luthar-Passi. The theory behind the method is briefly described in Chapter and also in the survey article .

The package uses the software 4ti2 and/or normaliz and hence is only properly working on systems which have at least one of the two installed. normaliz is usually automatically installed with the GAP-package NormalizInterface, see the documentation of that package for details. For more information on 4ti2 and to download it, please visit http://www.4ti2.de. To interact with these external software the package makes use of the 4ti2-Interface and normaliz-Interface written by S. Gutsche, C. Söger and M. Horn. The 4ti2-Interface in turn uses the package IO, that needs a C-part to be compiled; see the readme-file or the documentation of the IO-package for details. The package also provides the possibility to use redund from the lrslib software , to remove redundant inequalities before solving the system, which might speed up the computations significantly when 4ti2 is used. However, it is not a requirement to have lrslib installed. If the above mentioned preconditions are fulfilled, one just has to copy the HeLP-package in the GAP pkg-directory. Now the package can be loaded by typing LoadPackage("HeLP"); during a GAP-session. If the HeLP-package doesn't work properly on your computer, you might want to check Section for some trouble shooting.