Search All of the Math Forum:
Views expressed in these public forums are not endorsed by
Drexel University or The Math Forum.



Re: Haar measure on locally compact group
Posted:
Feb 6, 2013 10:45 AM


On Tue, 5 Feb 2013 14:17:15 0800 (PST), johnartin@gmail.com wrote:
>Hi, > >Can somebody help me by showing the outline or key steps to prove that if G is a locally compact group with the Haar >measure (just assume G is Abelian), then if the Haar measure of G is finite, then G must be compact. I tried many times, >got some progress but did not get to the final solution yet. Thanks
Must be in the standard books... ok, do this:
Say V is a neighborhood of 0 with compact closure. By continuity of the group operations there exists a nbd W of 0 with
W  W subset V.
Now let S be a subset of G maximal subject to the condition that
{x + W : w in S}
is pairwise disjoint.
Since W is open, m(W) > 0 and hence S is finite.
Now, for all x in G there exists y in S such that
(x + W) intersect (y + W) is nonempty.
This says that
y  x is in W  W,
hence y  x is in V, so that y is in x + V.
So G is the union of x + V for x in S, hence G is a finite union of compact sets, hence compact.
DU.



