These notes are based on lectures originally presented at the University of Uppsala in Sweden in the spring of 1994. The lectures were organised and financially made possible by Kersti Hermansson and Jörgen Tegenfeldt. Later I presented the same course at the University of Twente in Enschede in the Netherlands. I benefitted a lot from discussions with Mark Wojcik in Uppsala and Pierre van Sinderen, Marc in 't Panhuis, Reinier Akkermans and Johan Padding in Enschede.
The manuscript was typed by Pierre van Sinderen, who also did the calculations in chapter 2, together with Roelof de Vries. The final editing of the manuscript was done by Johan Padding, who also meticulously removed many errors, typing errors as well as insecurities from my side.
Some books and articles which have been very helpful in preparing these lectures are:
Wim Briels, Enschede, october 1998.