Abstract This paper is the fourth part of the publication ``Selected chapters
from algebra'', the first three having been published in previous
issues of the Teaching of Mathematics, Vol\. I (1998), 1--22, Vol\. II, 1
(1999), 1--30, Vol\. II, 2 (1999), 65--80, and Vol\. III, 1 (2000), 15--40.
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