   Products of Actions

### Products of Actions

Now we take two actions into account, GX and HY, say, and derive further actions from these. Without loss of generality we can assume X ÇY= Æ since otherwise we can rename the elements of X, in order to replace GX by a similar action GX', for which X' ÇY= Æ. Now we form the (disjoint) union X È Y and let G ´H act on this set as follows:
(G ´H) ´(X È Y) -> X È Y
where ((g,h),z) -> gz if z ÎX and ((g,h),z) -> hz if z ÎY. The corresponding permutation group will be denoted by bar (G) Åbar (H) (cf. the formula) and called the direct sum of bar (G) and bar (H). Another canonical action of G ´H is that on the cartesian product:
(G ´H) ´(X ´Y) -> X ´Y :((g,h),(x,y)) -> (gx,hy).
The corresponding permutation group will be denoted by bar (G) Äbar (H) and called the cartesian product of bar (G) and bar (H). An important particular case is
Example: Assume two finite and transitive actions of G on X and Y. They yield, as was just described, a canonical action of G ´G on X ´Y which has as one of its restrictions the action of D(G ´G), the diagonal, which is isomorphic to G, on X ´Y. We notice that, for fixed x ÎX, y ÎY, the following is true (see exercise):
• Each orbit of G on X ´Y contains an element of the form (x,gy).
• The stabilizer of (x,gy) is Gx ÇgGyg-1, hence the action of G on the orbit of (x,gy) is similar to the action of G on G/(Gx ÇgGyg-1) (recall the lemma).
• (x,gy) lies in the orbit of (x,g'y) if and only if
GxgGy=Gxg'Gy.

Hence the following is true:

Corollary: If G acts transitively on both X and Y, then, for fixed x ÎX, y ÎY, the mapping
G \\(X ´Y) -> Gx \G/Gy :G(x,gy) -> GxgGy
is a bijection (note that G \\(X ´Y) stands for D(G ´G) \\(X ´Y)). Moreover, the action of G on the orbit G(x,gy) and on the set of left cosets
G/(Gx ÇgGyg-1)
are similar. Hence, if D denotes a transversal of the set of double cosets Gx \G/Gy, for fixed x ÎX, y ÎY, then we have the following similarity:
G (X ´Y ) » G ( Èg ÎD G/(Gx ÇgGyg-1) ).

harald.fripertinger@kfunigraz.ac.at,
last changed: August 28, 2001   Products of Actions