1. Introduction 5
We may iterate this argument (for a fixed, but small, value of ε) and conclude
that
Φ(t0/2k)
=
exp(B/2k)
for all k = 0, 1, 2,... . By the homomorphism property and (1.1) we thus
have
Φ(qt0) = exp(qB)
whenever q is a dyadic rational, i.e., a rational of the form
a/2k
for some
integer a and natural number k. By continuity we thus have
Φ(st0) = exp(sB)
for all real s. Setting A := B/t0 we conclude that
Φ(t) = exp(tA)
for all real t, which gives existence of the representation and also real an-
alyticity and smoothness. Finally, uniqueness of the representation Φ(t) =
exp(tA) follows from the identity
A =
d
dt
exp(tA)|t=0.
Exercise 1.0.2. Generalise Proposition 1.0.1 by replacing the hypothesis
that Φ is continuous with the hypothesis that Φ is Lebesgue measurable.
(Hint: Use the Steinhaus theorem, see e.g. [Ta2011, Exercise 1.6.8].) Show
that the proposition fails (assuming the axiom of choice) if this hypothesis
is omitted entirely.
Note how one needs both the group-like structure and the weak reg-
ularity in combination in order to ensure the strong regularity; neither is
sufficient on its own. We will see variants of the above basic argument
throughout the course. Here, the task of obtaining smooth (or real ana-
lytic structure) was relatively easy, because we could borrow the smooth (or
real analytic) structure of the domain R and range Mn(C); but, somewhat
remarkably, we shall see that one can still build such smooth or analytic
structures even when none of the original objects have any such structure
to begin with.
Now we turn to a second illustration of the above principles, namely
Jordan’s theorem [Jo1878], which uses a discreteness hypothesis to upgrade
Lie type structure to nilpotent (and in this case, abelian) structure. We
shall formulate Jordan’s theorem in a slightly stilted fashion in order to
emphasise the adherence to the above-mentioned principles.
Theorem 1.0.2 (Jordan’s theorem). Let G be an object with the following
properties:
(1) (Group-like object) G is a group.
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