# Introduction to Mathematical Physics/Some mathematical problems and their solution/Particular trajectories and geometry in space phase

## Fixed points and Hartman theorem

editConsider the following initial value problem:

eqnl

with . It defines a flow: defined by .

By Linearization around a fixed point such that :

eql

The linearized flow obeys:

It is natural to ask the following question: What can we say about the solutions of eqnl based on our knowledge of eql?

**Theorem:**

If has no zero or purely imaginary eigenvalues, then there is a homeomorphism defined on some neighborhood of in locally taking orbits of the nonlinear flow to those of the linear flow . The homeomorphism preserves the sense of orbits and can be chosen to preserve parametrization by time.

When
has no eigen values with zero real part,
is called a *hyperbolic* or nondegenerate fixed point and the asymptotic
behaviour near it is determined by the linearization.

In the degenerate case, stability cannot be determined by linearization.

Consider for example:

Eigenvalues of the linear part are . If : a spiral sink, if : a repelling source, if a center (hamiltonian system).

## Stable and unstable manifolds

edit**Definition:**

The local stable and unstable manifolds and of a fixed point are

where is a neighborhood of the fixed point .

**Theorem:**

(Stable manifold theorem for a fixed point). Let be a hyperbolic fixed point. There exist local stable and unstable manifold and of the same dimesnion and as those of the eigenspaces , and of the linearized system, and tangent to and at . and are as smooth as the function .

An algorithm to get unstable and stable manifolds is given in ([#References|references]). It basically consists in finding an point sufficiently close to the fixed point , belonging to an unstable linear eigenvector space:

eqalphchoose

For continuous time system, to draw the unstable manifold, one has just to integrate forward in time from . For discrete time system, one has to integrate forward in time the dynamics for points in the segment where is the application.

The number in equation eqalphchoose has to be small enough for the linear approximation to be accurate. Typically, to choose one compares the distant between the images of given by the linearized dynamics and the exact dynamics. If it is too large, then is divided by 2. The process is iterated until an acceptable accuracy is reached.

## Periodic orbits

editIt is well known ([#References|references]) that there exist periodic (unstable) orbits in a chaotic system. We will first detect some of them. A periodic orbit in the 3-D phase space corresponds to a fixed point of the Poincar\'e map.

The method we choosed to locate periodic orbits is "the Poincare map" method ([#References|references]). It uses the fact that periodic orbits correspond to fixed points of Poincare maps. We chose the plane as one sided Poincare section. (The 'side' of the section is here defined by becoming positive)

Let us recall the main steps in locating periodic orbits by using the Poincare map method : we apply the Newton-Raphson algorithm to the application where is the Poincare map associated to our system which can be written as :

where denotes the set of the control parameters. Namely, the Newton-Raphson algorithm is here:

eqnewton

where is the Jacobian of the Poincare map evaluated in .

The jacobian of poincare map
needed in the scheme
of equation eqnewton is computed via
the integration of
the dynamical system:

where
is the Jacobian of
in
, and
is a Point of the Poincare section.
We chose a Runge--Kutta scheme, fourth order
([#References|references])
for the time integration of the whole previous system.
The time step was .

We have the relation:

where is the time needed at which the trajectory crosses le Poincare section again.

**Remark:**

Note that a good test for the accuracy of the integration is to check that on a periodic orbit, there is one eigenvalue of which is one.