# I can't tell the difference between these groups :/

1. Feb 7, 2010

### Firepanda

Z(G) = { x in G : xg=gx for all g in G } (center of a group G)

C(g) = { x in G : xg=gx } (centralizer of g in G)

I have to show both are subgroups, but what's the difference in the methods?

To me the first set is saying all the elements x1, x2,... in G when composed with every element in g, commute.

The second set tells me what x1, x2,... in G when composed with a single chosen element from g in G, commute.

Is this correct?

I've found the way to show Z(G) is a subgroup from here :

http://en.wikipedia.org/wiki/Center_(group_theory)#As_a_subgroup

So how does this differ from C(g), what other steps do I need?

Thanks

2. Feb 7, 2010

### Mandark

We say that two elements x, y of G commute if xy = yx. Suppose we have an element x of G, if it commutes with all other elements of G then it belongs to Z(G), which we call the center of G. Hence, Z(G) consists of all elements which commute with everything else. Notice that Z(G) depends only on G and doesn't refer to any specific element.

On the other hand C(g) depends on a specific element g in G. It's the set of all elements of G which commutes with g.

Try doing some examples.