1.1. Hilbert’s fifth problem 11
(1) (Group-like object) G is a topological group.
(2) (Weak regularity) G is locally Euclidean.
Then
(i) (Lie-type structure) G is isomorphic to a Lie group.
Exercise 1.1.2. Show that a locally Euclidean topological group is neces-
sarily Hausdorff (without invoking Theorem 1.1.9).
We will prove this theorem in Section 6. As it turns out, Theorem 1.1.9
is not directly useful for many applications, because it is often difficult to
verify that a given topological group is locally Euclidean. On the other
hand, the weaker property of local compactness, which is clearly implied by
the locally Euclidean property, is much easier to verify in practice. One can
then ask the more general question of whether every locally compact group
is isomorphic to a Lie group. Unfortunately, the answer to this question is
easily seen to be no, as the following examples show:
Example 1.1.10 (Trivial topology). A group equipped with the trivial
topology is a compact (hence locally compact) group, but will not be Haus-
dorff (and thus not Lie) unless the group is also trivial. Of course, this is
a rather degenerate counterexample and can be easily eliminated in prac-
tice. For instance, we will see later that any topological group can be made
Hausdorff by quotienting out the closure of the identity.
Example 1.1.11 (Infinite-dimensional torus). The infinite-dimensional
torus
(R/Z)N
(with the product topology) is an (additive) topological group,
which is compact (and thus locally compact) by Tychonoff’s theorem. How-
ever, it is not a Lie group.
Example 1.1.12 (p-adics). Let p be a prime. We define the p-adic norm
p
on the integers Z by defining n
p
:=
p−j,
where
pj
is the largest power
of p that divides n (with the convention 0
p
:= 0). This is easily verified to
generate a metric (and even an ultrametric) on Z; the p-adic integers Zp are
then defined as the metric completion of Z under this metric. This is easily
seen to be a compact (hence locally compact) additive group (topologically,
it is homeomorphic to a Cantor set). However, it is not locally Euclidean
(or even locally connected), and so is not isomorphic to a Lie group.
One can also extend the p-adic norm to the ring Z[
1
p
] of rationals of the
form
a/pj
for some integers a, j in the obvious manner; the metric com-
pletion of this space is then the p-adic rationals Qp. This is now a locally
compact additive group rather than a compact one (Zp is a compact open
neighbourhood of the identity); it is still not locally connected, so it is still
not a Lie group.
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