Originally published at 狗和留美者不得入内. You can comment here or there.
Let be a smooth real manifold. A smooth vector field
on
can be considered as a function from
to
. Every function
at every point
is by a vector field (which implicitly associates a tangent vector at every point) taken to some real value, which one can think of as the directional derivative of
along the tangent vector. Moreover, this varies smoothly with
.
Along any vector field, if we start at any point, we can trace a path along the vector field. Imagine a vector field in water based on the velocity that does not change with time. Take a point particle at a point at any time and we can deterministically predict its path both forward in time and backward in time. We call this an integral curve and it is easy to see that integral curves are equivalence classes.
On a manifold , at a point with chart
, under vector field
, we would have
where is the
th component of
and
. This is an ODE which is guaranteed to have a unique solution at least locally, and we assume for now that the parameter
can be maximally extended.
If we attach the initial condition that at , the integral curve is at
, and denote the coordinate by
, (1) becomes
,
Here, is called a flow generated by
, which necessarily satisfies
for any .
Within this is the structure of a one-parameter family where
(i) or
.
(ii) is the identity map.
(iii) .
We now ask the question how a smooth vector field changes along a smooth vector field
. If our manifold were simply
(with a single identity chart, globally) we would at any point
some direction along
and on an infinitesimal change along that,
would change as well. In this case, it is easy to represent tangent vectors with indexed coordinates. Naively, we could take the displacement in
, divide by the amount of displacement along
and take the limit. However, we have not defined addition of tangent vectors on different tangent spaces. To do so, we would need some meaningful correspondence between values on different tangent spaces. Why can we not simply do vector addition? Recall that tangent space elements are defined in terms of how they act on smooth functions from
to
instead of directly. It is only because they are linear in themselves with respect to any given such function that we can using vectors to represent them.
We resolve this in a more general fashion by defining the induced map on tangent spaces and
for smooth
between manifolds. Recall that an element of a tangent space is a map
(that also satisfies the Leibniz property:
). If
, then
. We define the induced map
in the following manner. If , then
, where
.
We notice how we can apply this on in our construction of the Lie derivative
of a vector field
with respect to vector field
. Since the flow is along
,
We define as the induced map of
.
If , then by definition,
.
That means
Using that by the chain rule,
,
we arrive at
Using the power series of at
, we get
Moreover, by (2),
Substituting (5) and (6) into (4) yields
There is a constant term, a first order term, and an . In (3), the constant term is subtracted out, and the
contributes nothing to the limit. This means that the Lie derivative is equal to the first order term, with
Notice how in (4), there is that we have omitted in (8). This is because we are using
as the basis of the tangent vector that is applied onto
.
We have in (8) what is the th component of the Lie bracket of
where