5. Chaos
In this section we will argue that analysis is part of local geometry. We begin by remarking that the derivative of a function in a point x where it is defined is the tangent of the angle between the strict transform of the blow up of the graph of the function with the space containing the graph. We now discuss relations between the derivatives of a function.
Given a partial differential equation
on a differentiable manifold
M where
, where
is algebraic and
3, and where
are coordinates we can consider the equation in question as the zero set of a algebraic map
from the restricted coordinate ring of the normal cone in a point to the real numbers.
We can consider the equation as the zero set of an algebraic map from the restricted coordinate ring of (some chart of) the exceptional divisor of the blow up of a point.
We can also consider the equation as the zero set of an algebraic map from the set of restricted regular functions on a tubular neighborhood of a point.
We can as well consider the equation as the zero set of an algebraic map on the -th tangent space (also known as the -th jet bundle) of M in the origin.
Here the restrictions are by the degree of the partial derivatives as defined by the differential equation in question. In other words, the zero set in question does not live in the entirety of the coordinate ring of the normal cone etc.
Finally we can formally speaking see the equation as an algebraic map of tangent vectors from one Zariski tangent space to another, disregarding whether these formal tangent spaces are the tangent spaces of something with even a differentiable structure. For instance we would see the Navier-Stokes equation as being an algebraic relationsship in four time-spatial dimensions and two dimensions . To solve the equation one would in this case only need to specify p and v.
In general given a closed neighborhood
of the origin in
we can ask for a solution
of the differential equation and we can ask for it’s stability and analycity. Suppose from now on that
and that the origin in
is an isolated critical point of
P. Then by [
8][
9] there is a
such that for each positive
,
are transverse. In particular there is an
such that for all
is a trivial
-fibration. In particular there exists smooth coordinates on
, in other words the partial differential equation
is satisfied after a small scalar perturbation. If the origin in
is a non-critical point one can additionally take
to be zero. In this case
is a
manifold and one can choose smooth coordinates on
, and again
is satisfied. Moreover since stability of the coordinates
is transversality of its multi jets to the orbits of a certain action by Mather [
7], Therom 4.1, it is in particular by Thoms Transversality Theorem [
9] an open condition. The dimensions where stability is dense is further specified by Mather
Example 1 (The Navier-Stokes equations).
The Navier-Stokes equations read
The map are defined by the columns and one asks whether is a regular value. In standard coordinates on the Navier-Stokes equations define the intersection of 3 linear hyperplanes and 3 quadrics in 27 variables. 3 of the variables can be eliminated from the last 3 equations leaving 3 quadrics in 24 variables. Then there are 9 more equations of the form and 3 equations and 9 equations of the form leaving us with 3 quadrics in unknowns.
Because of the linear terms the origin is a non-critical point of P so we deduce that there exists smooth coordinate functions on ; in particular the Navier-Stokes equations are satisfied for and the solutions are analytic. Moreover the solutions for are stable since transversality is a stable property by Thom’s (weak) Transversality Theorem [6] because the origin is non-critical which means that . This proves the first part of the Navier-Stokes problem. We remark that if the external force is chosen such that f is a critical value of P one cannot deduce analycity of the solution4.
Given a dynamical system
depending on
for
T a real analytic manifold and
a smooth manifold we consider
and it’s restriction to the real analytic set
given by
and we can always find a Whitney stratification such that
is locally trivial above each stratum. In particular the topological type of
and
are the same whenever
and
belong to the same stratum. Indeed by Sard’s Theorem [
6] the set of critical values is of measure zero and one can apply the Isotopy Lemma of Thom-Mather [
6] to its complement. In particular if
is a solution over
then there exist a stratum preserving homeomorphism
such that
and
is then a solution over
which is topologically equivalent to
.
Definition 1. One says that undergoes major chaos whenever a solution is mapped to a non-topologically equivalent solution when α changes from one stratum to another.
Definition 2. One says that undergoes minor chaos whenever a solution is mapped to a non-topologically equivalent solution when α changes within a stratum.
Definition 3. One says that undergoes chaos if it undergoes minor or major chaos under perturbation of the parameter.
Definition 4. The real analytic set is called the Milnor fiber of the dynamical system
It is the main object of study in the understanding of chaos. The following theorem says that all chaos in dimension n lives in a manifold of dimension n.
Theorem 4. A system undergoes chaos if and only the outward unit normal to traces out a curve which is non-topologically equivalent to the curve traced out by the outward unit normal to .
Proof. We can always assume that the trajectories to are parametrised by unit speed, since M is a smooth manifold. That are paths on the Milnor fiber then gives the claim. That one can work instead with the unit normal is because the normal space is orthogonal and of the same dimension. □
An interesting form of chaos is the case of a perturbed radial vector field in local coordinates on M.
Definition 5. One says that undergoes spherical chaos if it undergoes chaos.
The main result in this section is that in the simplest case where the dimension is one chaos is either spherical or lives in dimension zero.
Theorem 5.
Suppose that , that has only isolated singular points and that the hypothesis of [?, Theorem 5]) hold. There is a stratum preserving diffeomorphism
In particular chaos either is spherical or appear on the boundary of the Milnor fiber.
Proof. The proof is identical to the proof of [
?, Theorem 5] except that in the last step one uses that
by a stratum-preserving diffeomorphism, which follows from Ehresmann’s Fibration Theorem [
6]. In the same way
by a stratum-preserving diffeomorphism. Then one identifies the strata one wants and get the wanted stratum-preserving diffeomorphism. For the last step one simply remarks that the Milnor fiber of
is identical to the Milnor fiber of
. □
Definition 6. A stationary point of a dynamical system is called a differential singular point.
Definition 7. A critical point of a function is called a topological singular point.
It would be interesting to ask whether this result generalises to when has non isolated critical points. It turns out that apart from boundary chaos and spherical chaos one also gets what one might call tubular chaos. As before the Milnor fiber in the above sense is that of the function .
Definition 8. One says that undergoes tubular chaos if undergoes chaos.
Lemma 2. If as a isolated critical value in the origin and satisfies the transversality condition of D.B Massey and if then chaos of consists of boundary chaos of V, tubular and boundary chaos of and spherical chaos.
Proof. Apply [Theorem 2]LarsIV and Lemma 5 to obtain that the chaos consists of chaos along the link of
V, chaos along the link of
, and chaos along
. The first two are thus boundary chaos and the latter is clearly tubular chaos and boundary chaos again by [Theorem 2]LarsIV and Lemma 5 by the definition (for
varying in
. Then there is only singular chaos corresponding to chaos at
. But
is non-chaotic. □
When the unit normal undergoes change of parameter the Milnor fiber ”breathes” intuitively speaking and one can then ask what kind of figures are possible? For instance for spherical chaos is a sphere and when changes one can in the case of get a ellipsoid or a one-sheeted hyperboloid and a two-sheeted hyperboloid where for simplicity Then if t is fixed at varies then there is only linear motion possible. So here we have the most trivial Milnor fibers and only linear motion. On the other hand for which corresponds to a cusp singularity (see [? ?] for the homology groups of -singularities in general) we can guess that a small change in parameter gives a larger effect.
Therefore, to measure the amount of chaos persistent we consider a small ball intersecting the Milnor fibers transversally for and for .
Definition 9.
The first energy measure of chaos of a solution is
Definition 10.
The second energy measure of chaos of a solution is
Here the determinant appears as the natural measure of size of a matrix in the first definition and in the second definition where Let and denote the local Gaussian curvature respectively Euler characteristics.
Proof. For the first claim this is just the Gauss-Bonnet theorem stated locally. For the second claim one uses the exponential map to transport the question to the tangent space of the Milnor fiber in question, then one integrates on the tangent spaces giving the volume in the first summand and then in the second summand one again gets the volume since one is just integrating the ball in a normal direction. □
We now come to the main result which is the classification result mentioned in the abstract.
Theorem 6 (Classification Theorem of Chaos). Chaos in dimension n is one dimensional and occurs on the -dimensional Milnor fiber of a function on which is either singular isolated, singular non-isolated or non-singular. In the first case this chaos is either boundary chaos or spherical chaos. In the second case this chaos is either boundary chaos, spherical chaos or tubular chaos. In the third chaos this chaos is tubular chaos.