<p>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 <a href="chapBib_mj.html#biBLP">[LP89]</a> and which was extended by M. Hertweck in <a href="chapBib_mj.html#biBHertweckBrauer">[Her07]</a>. These names also constitute the name of the method, suggested by O. Konovalov: <em>He</em>rtweck-<em>L</em>uthar-<em>P</em>assi. The theory behind the method is briefly described in Chapter <a href="chap5_mj.html#X84AF2F1D7D4E7284"><span class="RefLink">5</span></a> and also in the survey article <a href="chapBib_mj.html#biBBaMaHeLPArticle">[BM18]</a>.</p>
<p>The package uses the software 4ti2 <a href="chapBib_mj.html#biB4ti2">[tea]</a> and/or normaliz <a href="chapBib_mj.html#biBNormaliz">[BIR+]</a> 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 <span class="URL"><a href="https://4ti2.github.io">4ti2.github.io</a></span>. 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 <a href="chapBib_mj.html#biBlrslib">[Avi]</a>, 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 <code class="keyw">LoadPackage("HeLP");</code> during a GAP-session. If the HeLP-package doesn't work properly on your computer, you might want to check Section 6.1 for some trouble shooting.
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