# Periodic orbits computation

Consider the Cauchy problem

$$$\frac{du}{dt}=F(u,p).$$$

A periodic solution with period $T$ satisfies

\begin{align} \frac{du}{dt}&=F(u,p)\\ u(0)&=u(T). \end{align}

We provide 4 methods for computing periodic orbits (PO):

1. one (Trapezoid) based on finite differences to discretize a Cauchy problem,
2. one (Collocation) based on orthogonal collocation to discretize a Cauchy problem,
3. two (Shooting) based on the flow associated to a Cauchy problem.

It is important to understand the pros and cons of each method to compute PO in large dimensions.

In small dimensions, the collocation method is the preferred choice. It is the most precise, the fastest with dedicated linear solvers, time adaptive and feature full.

### Trapezoid method

The Trapezoid method (or the Collocation one) is usually faster than the ones based on Shooting but it requires more memory as it saves the whole orbit. However the main drawback of this method is that the associated linear solver is not "nice", being composed of a cyclic matrix for which no generic Matrix-free preconditioner is known. Hence, the Trapezoid method is often used with an ILU preconditioner which is severely constrained by memory. Also, when the period of the cycle is large, finer time discretization (or mesh adaptation which is not yet implemented) must be employed which is also a limiting factor both in term of memory and preconditioning.

### Collocation method

The Collocation method is (for now) the slowest of the three methods implemented for computing periodic orbits in large dimensions. However, it is by far the most precise one. Additionally, the mesh can be automatically adapted during the continuation. The implementation will be improved for large dimensional systems like the Trapezoid method one.

### Shooting method

The methods based on Shooting do not share the same drawbacks because the associated linear system is usually well conditioned, at least in the simple shooting case. They are thus often used without preconditioner at all. Even in the case of multiple shooting, this can be alleviated by a simple generic preconditioner based on deflation of eigenvalues (see Linear solvers (LS)). Also, the time stepper will automatically adapt to the stiffness of the problem, putting more time points where needed unlike the method based on finite differences which requires an adaptive (time) meshing to provide a similar property. Finally, we can use parallel Shooting to greatly decrease the computation time.

The main drawback of the method is to find a fast time stepper, at least to compete with the method based on finite differences. The other drawback is the precision of the method which cannot compete with the collocation one.

## Important notes

We regroup here some important notes which are valid for all methods above.

### 1. Accessing the periodic orbit

In record_from_solution, plot_solution or after the computation of a branch of periodic orbits, how do I obtain the periodic orbit, for plotting purposes for example? If x is the solution from newton for the parameter p, you can obtain the periodic orbit as follows

xtt = get_periodic_orbit(x, p)

where xtt.t contains the time mesh and xtt[:,:] contains the different components. Note that for Trapezoid and collocation methods, calling get_periodic_orbit is essentially free as it is a reshape of x. However, in the case of Shooting methods, this requires recomputing the periodic orbit which can be costly for large scale problems.

### 2. Finaliser

If you pass a finalise_solution function to continuation, the following occurs:

1. If the newton solve was successful, we update the phase condition every update_section_every_step
2. we call the user defined finalizer finalise_solution

### 3. Record from solution

You can pass your own function to continuation. In the particular case of periodic orbits, the method is called like record_from_solution(x, opt; k...) where opt.p is the current value of the continuation parameter and opt.prob is the current state of the continuation problem. You can then obtain the current periodic orbit using (see above)

xtt = get_periodic_orbit(x, opt.p)

### 4. Plot solution

Similarly to record_from_solution, the method is called like plot_solution(x, opt; k...) where opt.p is the current value of the continuation parameter and opt.prob is the current state of the continuation problem.

### 5. Most precise method for Floquet coefficients

The state of the art method is based on a Periodic Schur decomposition. It is available through the package PeriodicSchurBifurcationKit.jl. For more information, have a look at FloquetPQZ.

### 6. Misc

set_params_po generate_ci_problem generate_solution