Finite symmetric groupsActionsActions of groupsBilateral classes, symmetry classes of mappings

Bilateral classes, symmetry classes of mappings

From a given action we can derive various other actions in a natural way, e.g. GX yields bar (G)X, bar (G) being the homomorphic image of G in SX, which was already mentioned. We also obtain the subactions   GM on subsets M ÍX which are nonempty unions of orbits. Furthermore there are the restrictions  

UX to the subgroups U of G. As the orbits of GX are unions

of orbits of UX, the comparison of actions and restrictions is a suitable way of generalizing or specializing structures if they can be defined as orbits. The following example will show what is meant by this.

Examples: Let U denote a subgroup of the direct product G ´G. Then U acts on G as follows:
U ´G -> G :((a,b),g) -> agb-1.
The orbits U(g)= {agb-1 | (a,b) ÎU } of this action are called the bilateral classes of G with respect to U. By specializing U we obtain various interesting group theoretical structures some of which have been mentioned already:

Hence left and right cosets, double cosets and conjugacy classes turn out to be special cases of bilateral classes. Being orbits, two of them are either equal or disjoint, moreover, their order is the index of the stabilizer of an element. We have mentioned this in connection with conjugacy classes and centralizers of elements, here is the consequence for double cosets: Since

(A ´B)g= {(gbg-1,b) | b ÎB },

we obtain


harald.fripertinger "at" uni-graz.at http://www-ang.kfunigraz.ac.at/~fripert/
UNI-Graz Institut für Mathematik
UNI-Bayreuth Lehrstuhl II für Mathematik
last changed: January 19, 2005

Finite symmetric groupsActionsActions of groupsBilateral classes, symmetry classes of mappings