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<Chapter Label="Chapter_Introduction">
<Heading>Introduction</Heading>
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 <Cite Key="LP"/> and which was
extended by M. Hertweck in <Cite Key="HertweckBrauer"/>. These names also constitute the name
of the method, suggested by O. Konovalov: <Emph>He</Emph>rtweck-<Emph>L</Emph>uthar-<Emph>P</Emph>assi.
The theory behind the method is briefly described in Chapter <Ref Chap='Chapter_Background'/>
and also in the survey article <Cite Key="BaMaHeLPArticle"/>. <P/>
<P/>
The package uses the software 4ti2 <Cite Key="4ti2"/> and/or normaliz
<Cite Key="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
<URL Text="4ti2.github.io">https://4ti2.github.io</URL>.
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 <Cite Key="lrslib"/>,
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 <K>LoadPackage("HeLP");</K> during a GAP-session.
If the HeLP-package doesn't work properly on your computer, you might want to check Section
<Ref Sect='Chapter_Remarks_on_technical_problems_and_the_implementation_Section_Making_the_HeLP-package_run'/>
for some trouble shooting.
</Chapter>
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