Introduction to Mathematical Physics/Some mathematical problems and their solution/Particular trajectories and geometry in space phase
Fixed points and Hartman theorem edit
Consider the following initial value problem:
with . It defines a flow: defined by .
By Linearization around a fixed point such that :
The linearized flow obeys:
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
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:
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 untill an acceptable accuracy is reached.
Periodic orbits edit
It 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:
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.