This section overviews the various integrators available from DynamicalSystemsBase, as well as gives some insight into the internals, so that other developers that want to use this library can build upon it.

## Integrators

DynamicalSystemsBase.integratorFunction
integrator(ds::DynamicalSystem [, u0]; diffeq...) -> integ

Return an integrator object that can be used to evolve a system interactively using step!(integ [, Δt]). Optionally specify an initial state u0.

The state of this integrator is a vector.

• diffeq... are keyword arguments propagated into init of DifferentialEquations.jl. See trajectory for examples. Only valid for continuous systems.
DynamicalSystemsBase.parallel_integratorFunction
parallel_integrator(ds::DynamicalSystem, states; kwargs...)

Return an integrator object that can be used to evolve many states of a system in parallel at the exact same times, using step!(integ [, Δt]).

states are expected as vectors of vectors.

Keyword Arguments

• diffeq... : Keyword arguments propagated into init of DifferentialEquations.jl. See trajectory for examples. Only valid for continuous systems. These keywords can also include callback for event handling.

It is heavily advised to use the functions get_state and set_state! to manipulate the integrator. Provide i as a second argument to change the i-th state.

DynamicalSystemsBase.tangent_integratorFunction
tangent_integrator(ds::DynamicalSystem, Q0 | k::Int; kwargs...)

Return an integrator object that evolves in parallel both the system as well as deviation vectors living on the tangent space, also called linearized space.

Q0 is a matrix whose columns are initial values for deviation vectors. If instead of a matrix Q0 an integer k is given, then k random orthonormal vectors are choosen as initial conditions.

Keyword Arguments

• u0 : Optional different initial state.
• diffeq... : Keyword arguments propagated into init of DifferentialEquations.jl. See trajectory for examples. Only valid for continuous systems. These keywords can also include callback for event handling.

It is heavily advised to use the functions get_state, get_deviations, set_state!, set_deviations! to manipulate the integrator.

Description

If $J$ is the jacobian of the system then the tangent dynamics are the equations that evolve in parallel the system as well as a deviation vector (or matrix) $w$:

\begin{aligned} \dot{u} &= f(u, p, t) \\ \dot{w} &= J(u, p, t) \times w \end{aligned}

with $f$ being the equations of motion and $u$ the system state. Similar equations hold for the discrete case.

Notice that the state type integrator.u of each integrator is quite different and does change between the possible versions of a DynamicalSystem!

## Integrator state functions

There are four functions associated with the integrators that we export:

DynamicalSystemsBase.get_stateFunction
get_state(ds::DynamicalSystem)

Return the state of ds.

get_state(integ [, i::Int = 1])

Return the state of the integrator, in the sense of the state of the dynamical system.

If the integrator is a parallel_integrator, passing i will return the i-th state. The function also correctly returns the true state of the system for tangent integrators.

DynamicalSystemsBase.set_state!Function
set_state!(integ, u [, i::Int = 1])

Set the state of the integrator to u, in the sense of the state of the dynamical system. Works for any integrator (normal, tangent, parallel).

For parallel integrator, you can choose which state to set (using i).

Automatically does u_modified!(integ, true).

Note

These functions work with any possible integrator and it is best to use the to change states robustly!

## Re-initializing an integrator

It is more efficient to re-initialize an integrator using reinit! than to create a new one. This can be very helpful when looping over initial conditions and/or parameter values.

All high-level functions from ChaosTools have a set-up part that creates an integrator, and a low-level part that does the computation. The low level part is your friend! Use it! See the Using GALI page for an example as well as the section below.

The reinit! call signature is the same for continuous and discrete systems. In the following, state is supposed to be a D dimensional vector (state of the dynamical system).

1. reinit!(integ, state) : to be used with standard integrator.
2. reinit!(integ, Vector_of_states) : to be used with the parallel_integrator.
3. reinit!(integ, state, Q0::AbstractMatrix) : to be used with the tangent_integrator. This three argument version of reinit! is exported from DynamicalSystemsBase.

### Re-init of continuous tangent integrator

Here we compute the lyapunovspectrum for many different initial conditions.

ds = Systems.lorenz()
tinteg = tangent_integrator(ds, 2)
ics = [rand(3) for i in 1:100]
for ic in ics
reinit!(tinteg, ic, orthonormal(3, 2))
λ = lyapunovspectrum(tinteg, 1000, 0.1, 10.0)
# reminder: lyapunovspectrum(tinteg, N, dt::Real, Ttr::Real = 0.0)
end

### Re-init of discrete parallel integrator

Here we compute the lyapunov for many different parameters.

ds = Systems.henon()
u0 = rand(SVector{2})
ps = 1.2:0.01:1.4
pinteg = parallel_integrator(ds, [u0, u0 + 1e-9rand(SVector{2})])
for p in ps
set_parameter!(ds, 1, p)
reinit!(pinteg, [u0, u0 + 1e-9rand(SVector{2})])
λ = lyapunov(pinteg, 1000, 10, 1, 1e-9, 1e-6, 1e-12)
# reminder: lyapunov(pinteg, T, Ttr, dt, d0, ut, lt)
end

## Using callbacks with integrators

For the case of continuous systems you can add callbacks from the event handling of DifferentialEquations.jl. This is done simply as a keyword argument to the initializers.

In this example we use a simple SavingCallback to save the distance between the two states of a parallel_integrator.

using DynamicalSystems, DiffEqCallbacks
using LinearAlgebra: norm

kwargs = (abstol=1e-14, reltol=1e-14, maxiters=1e9)
ds = Systems.lorenz()
d0 = 1e-9
T = 100.0

save_func(u, t, integrator) = norm(u[1] - u[2])
saved_values = SavedValues(eltype(ds.t0), eltype(get_state(ds)))
cb = SavingCallback(save_func, saved_values)

u0 = get_state(ds)
pinteg = parallel_integrator(ds, [u0, u0 + rand(SVector{3})*d0*√3];
kwargs..., callback = cb)
step!(pinteg, T)
t = saved_values.t
n = saved_values.saveval
Float64[]

As expected you can see that the recorded distance between two states is increasing.

## DynamicalSystem implementation

abstract type DynamicalSystem{
IIP,     # is in place , for dispatch purposes and clarity
S,       # state type
D,       # dimension
F,       # equations of motion
P,       # parameters
JAC,     # jacobian
JM,      # jacobian matrix
end
The DynamicalSystem stores only the absolutely necessary information. Every other functionality of DynamicalSystems.jl initializes an integrator.
The final type-parameter IAD is useful when creating the tangent_integrator, so that the vector field is not computed twice!