Abstract
In the last paragraph we have constructed elements of Г (for a special case only, but it is obvious how to generalize the procedures), but no attempt was made to prove that the elements so obtained actually generate Г or at least a subgroup of finite index. To the best of my knowledge, this has been achieved for no such Г, if d ≥ 3. Paradoxically enough, generation of a cofinite subgroup becomes trivial if we pass from Λ to Mn (Λ), n ≥ 2: by results of K-theory, the elementary matrices will do the job; see e.g. [BRi] and the references quoted there. In the quaternion case d = 2, an algorithm is given in Fricke-Klein [FK], as well as many examples ([FK], p. 539 ss); marvellously drawn are the tesselations of the upper half plane resulting from the explicit fundamental domains and their translates.
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© 2000 Springer Basel AG
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Kleinert, E. (2000). Problems. In: Units in Skew Fields. Progress in Mathematics, vol 186. Birkhäuser, Basel. https://doi.org/10.1007/978-3-0348-8409-9_9
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DOI: https://doi.org/10.1007/978-3-0348-8409-9_9
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