In this entry, we shall discuss the following question: “What is a gradient flow?”.
Let be a Hilbert space with norm . Let be a convex, lower semi-continuous functional defined on a dense domain .
Definition. The subdifferential of at a point is the set defined by
.
Vector is called subgradient at , thus, the set of all subgradients at is called the subdifferential at .
The geometric meaning of subdifferential is as the set of all possible “slopes” of affine hyperplanes touching the graph of from below at the point . Thus, is differentiable at iff its subdifferential at contains exactly one vector as its derivative at that point.
We now recall the classical definition of gradient flow on a Hilbert space.
Definition. A function , the class of absolutely continuous from to , is a gradient flow of the convex, lower semi-continuous functional iff the differential inclusion
is satisfied almost everywhere with respect to .
In practice for a given flow
,
if is differentiable and is given as an integral over some domain, say, , we simply verify there is some number so that
.
If , the flow is called positive gradient flow. Now we discuss some examples.
Example 1. The following semilinear heat equation
corresponds formally to the gradient flow associated to the energy functional
.
Here we assume the boundary and initial conditions are all zero just for simplicity.
Observe that
and
.
Therefore
i.e.
is the gradient flow associated to the energy functional
.
Example 2. Heat flow for Nirenberg’s problem
is also a gradient flow for the following functional
.
For the details of this flow, we refer the reader to a paper due to Michael Struwe published in Duke Math. J. in 2005 [here].
Example 3. -curvature flow
is a gradient flow for some functional. For details, we refer the reader to a paper due to Simon Brendle published in Ann. of Math. in 2003 [here].
Source: Steepest descent flows and applications to spaces of probability measures by Luigi Ambrosio.
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