GX simeq HY,in order to indicate the existence of such a pair of mappings. If G=H we call GX and GY similar actions, if and only if they are isomorphic by ( h, q), where moreover h= id G, the identity mapping (cf. exercise). We indicate this by
GX » GY.An important special case follows directly from the proof:
Lemma: If GX is transitive then, for any x ÎX, we have thatGX » G(G/Gx).
A weaker concept is that of G- homomorphy. We shall write
GX ~ GYif and only if there exists a mapping q:X -> Y which is compatible with the action of G: q(gx)=g q(x). Later on we shall see that the use of G-homomorphisms is one of the most important tools in the constructive theory of discrete structures which can be defined as orbits of groups on finite sets. A characterization of G-homomorphy gives
Lemma: Two actions GX and GY are G-homomorphic if and only if for each x ÎX there exist y ÎY such that Gx ÍGy.
Proof: In the case when q:X -> Y is a G-homomorphism, then Gx ÍG q(x) since, for each g ÎGx,
q(x)= q(gx)=g q(x).On the other hand, if for each x ÎX there exist y ÎY such that Gx ÍGy, we can construct a G-homomorphism in the following way: Assume a transversal T(G \\X) of the orbits, and choose, for each element t of this transversal, an element yt ÎY such that Gt ÍGyt. An easy check shows that
q:X -> Y :gt -> gytis a well defined mapping and also a G-homomorphism.