I am working on problem 4.1.9 in Dummit and Foote's Abstract Algebra. Thought I am in an algebra course, we don't use this book, and this isn't a homework assignment.
G acts transitively on a finite set A and H is a normal subgroup of G. are the distinct orbits of H on A.
I have already done half of the problem, which is to show all orbits of H have the same cardinality and that G is transitive on the set where G acts on this set via representative.
My problem is this: Prove that if then and prove that .
I cannot do the second part (the first isn't hard). A hint is to use the 2nd isomorphism theorem, but I don't see how it helps.
Actually, I decided to ignore the advice about the 2nd isomorphism theorem and I thought of the following map that I think is a bijection.
the orbit containing g(a), where a is given in the problem.
The map is definitely surjective. It is well defined because if you have two cosets and then it must be true that for elements and
where the 2nd equality is because g_a is a stabilizer and the last equality because in part (a), it is shown that any element of G permutes the different orbits, taking all elements in one orbit to all elements in another...
Finally it is injective because if then
This seems to work... perhaps the hint was a red-herring (or perhaps I made a mistake).