Residue-class-wise affine group
In mathematics, specifically in group theory, residue-class-wise affine
groups are certain permutation groups acting on
(the integers), whose elements are bijective
residue-class-wise affine mappings.
A mapping is called residue-class-wise affine
if there is a nonzero integer
such that the restrictions of
to the residue classes
(mod
) are all affine. This means that for any
residue class
there are coefficients
such that the restriction of the mapping
to the set
is given by
.
Residue-class-wise affine groups are countable, and they are accessible
to computational investigations.
Many of them act multiply transitively on or on subsets thereof.
A particularly basic type of residue-class-wise affine permutations are the
class transpositions: given disjoint residue classes
and
, the corresponding class transposition is the permutation
of
which interchanges
and
for every
and which
fixes everything else. Here it is assumed that
and that
.
The set of all class transpositions of generates
a countable simple group which has the following properties:
- It is not finitely generated.
- Every finite group, every free product of finite groups and every free group of finite rank embeds into it.
- The class of its subgroups is closed under taking direct products, under taking wreath products with finite groups, and under taking restricted wreath products with the infinite cyclic group.
- It has finitely generated subgroups which do not have finite presentations.
- It has finitely generated subgroups with algorithmically unsolvable membership problem.
- It has an uncountable series of simple subgroups which is parametrized by the sets of odd primes.
It is straightforward to generalize the notion of a residue-class-wise affine group
to groups acting on suitable rings other than ,
though only little work in this direction has been done so far.
See also the Collatz conjecture, which is an assertion about a surjective, but not injective residue-class-wise affine mapping.
References and external links
- Stefan Kohl. Restklassenweise affine Gruppen. Dissertation, Universität Stuttgart, 2005. Archivserver der Deutschen Nationalbibliothek OPUS-Datenbank(Universität Stuttgart)
- Stefan Kohl. RCWA – Residue-Class-Wise Affine Groups. GAP package. 2005.
- Stefan Kohl. A Simple Group Generated by Involutions Interchanging Residue Classes of the Integers. Math. Z. 264 (2010), no. 4, 927–938.