For a) I proved the subgroup property. My problem is the example. H clearly has to be a group of some kind else the statement "H is not a subgroup of N_G(H)" is empty of meaning. So H has to be a group, but not a subgroup of G. I can't image how this could arise. Any hints?Let H be a subgroup of a group G.
a) Show that . Give an example to show that this is not necessarily true if H is not a subgroup of G.
b) Show that if and only if H is abelian.
For b) I have proven that if H is abelian then . I am having trouble with "If implies H is abelian. If were abelian it would be easy, but I don't think this is true.
Thanks for any help!