# Collocation for finding periodic orbits of ODEs 2020-05-01

Every now and again I'm asked how to compute the periodic orbits of ODEs using a boundary value solver. Each time, I go looking for old code that does this and, each time, I can't find it and end up rewriting the collocation code from scratch.

This time I thought I'd put my code here so that I have a better chance of finding it again in the future!

The basic idea is to use a Fourier differentiation matrix to approximate the derivatives along the orbit and use a nonlinear solver to ensure that those derivatives match the vector field. If you want to know more about these types of spectral methods, take a look at the excellent (and short!) introduction by Trefethen in "Spectral Methods in MATLAB", SIAM 2000. If you want more detail then the magnum opus by Boyd "Chebyshev and Fourier Spectral Methods", Dover 2001 (freely available on his personal website) is also very good.

Nowadays, my preference is for coding in Julia - it's very clean and flexible. Here is the code (which could be better!).

``````# Released under the MIT expat license by David A.W. Barton (david.barton@bristol.ac.uk) 2020
using StaticArrays
using NLsolve
using OrdinaryDiffEq

"""
duffing(u, p, t)

The vector field of the forced Duffing equation.
"""
duffing(u, p, t) = SVector(u, p.Γ*sin(p.ω*t) - 2p.ξ*u - p.ωₙ^2*u - p.β*u^3)

"""
fourier_diff([T=Float64,] N; order=1)

Create a Fourier differentiation matrix of the specified order with numerical type T on the
domain `x = LinRange{T}(0, 2π, N+1)[1:end-1]`.
"""
function fourier_diff(T::Type{<:Number}, N::Integer; order=1)
D = zeros(T, N, N)
n1 = (N - 1) ÷ 2
n2 = N ÷ 2
x = LinRange{T}(0, π, N+1)
if order == 1
for i in 2:N
sgn = (one(T)/2 - iseven(i))
D[i, 1] = iseven(N) ? sgn*cot(x[i]) : sgn*csc(x[i])
end
elseif order == 2
D[1, 1] = iseven(N) ? -N^2*one(T)/12 - one(T)/6 : -N^2*one(T)/12 + one(T)/12
for i in 2:N
sgn = -(one(T)/2 - iseven(i))
D[i, 1] = iseven(N) ? sgn*csc(x[i]).^2 : sgn*cot(x[i])*csc(x[i])
end
else
error("Not implemented")
end
for j in 2:N
D[1, j] = D[N, j-1]
D[2:N, j] .= D[1:N-1, j-1]
end
return D
end
fourier_diff(N::Integer; kwargs...) = fourier_diff(Float64, N; kwargs...)

"""
collocation_setup(u)

Return a data structure used internally by the `collocation!` function. `u` should be a
matrix with states down the columns and time across the rows (used for size/type information
only).
"""
function collocation_setup(u::AbstractMatrix)
return (ndim=size(u, 1), nmesh=size(u, 2), Dt=-fourier_diff(eltype(u), size(u, 2))*2π)
end

"""
collocation!(res, f, u, p, T, coll)

Calculate the residual of the collocation equations using a Fourier discretisation. Assumes
that a phase condition is not required (i.e., the equations are non-autonomous or the period
is known).

# Arguments
- `res`: residual (mutated)
- `f`: vector field function (expected to take the arguments (u, p, t))
- `u`: state variables along the orbit (vector)
- `p`: parameter vector passed to the vector field function
- `T`: period of oscillation
- `coll`: the output of `collocation_setup`

# Returns
- `res`: residual
"""
function collocation!(res, f, u, p, T, coll)
# Matrix of derivatives along the orbit
D = reshape(u, (coll.ndim, coll.nmesh))*coll.Dt
ii = 1:coll.ndim
for i in 1:coll.nmesh
# Subtract the desired derivative from the actual derivative
res[ii] .= D[ii] .- T.*f(u[ii], p, T*(i-1)/coll.nmesh)
ii = ii .+ coll.ndim
end
return res
end

function example(; nmesh=20)
p = (Γ=0.1, ω=1.0, ξ=0.05, ωₙ=1.0, β=0.1)
# Do initial value simulation to get a reasonable starting point
prob = ODEProblem(duffing, SVector(0.0, 0.0), (0.0, 100*2π/p.ω), p)
odesol = solve(prob, Tsit5())
# Refine using collocation
t = range(0, 2π/p.ω, length=nmesh+1)[1:end-1]
uvec = reinterpret(Float64, odesol(99*2π/p.ω .+ t).u)
umat = reshape(uvec, (:, nmesh))
coll = collocation_setup(umat)
nlsol1 = nlsolve((res, u) -> collocation!(res, duffing, u, p, 2π/p.ω, coll), uvec)
# Adjust the parameters slightly (actually quite a bit!) and correct
p = (Γ=0.1, ω=1.1, ξ=0.05, ωₙ=1.0, β=0.1)
nlsol2 = nlsolve((res, u) -> collocation!(res, duffing, u, p, 2π/p.ω, coll), uvec)
return (nlsol1, nlsol2)
end

function plot_example()
# Needs `using Plots` or similar
nmesh = 20
# The two solutions don't actually have the same period but normalize to [0, 2π]
t = linspace(0, 2π, length=nmesh+1)[1:end-1]
(sol1, sol2) = example()
plot(t, sol1.zero[1:2:end])
plot!(t, sol2.zero[1:2:end])
end``````

If you insist on using Matlab, the translation of the Julia code is below. Note that this uses the `fourdif` function by Reddy and Weideman to generate the Fourier differentiation matrix. (Also note that this can be put in a single file called `fourier_collocation.m`.)

``````function [nlsol1, nlsol2] = fourier_collocation()
% FOURIER_COLLOCATION Implement Fourier collocation for an arbitrary autonomous
% ODE. Assumes that the equations are non-autonomous or the period is known.

% Released under the MIT expat license by David A.W. Barton (david.barton@bristol.ac.uk) 2020

nmesh = 20;
p = struct('Gamma', 0.1, 'omega', 1.0, 'xi', 0.05, 'omegan', 1.0, 'beta', 0.1);
% Do initial value simulation to get a reasonable starting point
sol = ode45(@(t, u)duffing(t, u, p), [0, 100*2*pi/p.omega], [0, 0]);
% Refine using collocation
t = linspace(0, 2*pi/p.omega, nmesh+1);
t = t(1:end-1);
umat = deval(sol, 99*2*pi/p.omega + t);
uvec = umat(:);
coll = collocation_setup(umat);
nlsol1 = fsolve(@(u)collocation(@duffing, u, p, 2*pi/p.omega, coll), uvec)
% Adjust the parameters slightly (actually quite a bit!) and correct
p = struct('Gamma', 0.1, 'omega', 1.1, 'xi', 0.05, 'omegan', 1.0, 'beta', 0.1);
nlsol2 = fsolve(@(u)collocation(@duffing, u, p, 2*pi/p.omega, coll), uvec)

plot(t, nlsol1(1:2:end), 'b', t, nlsol2(1:2:end), 'r');

end

function du = duffing(t, u, p)
du = [u(2); p.Gamma*sin(p.omega*t) - 2*p.xi*u(2) - p.omegan^2*u(1) - p.beta*u(1)^3];
end

function coll = collocation_setup(u)
[~, D] = fourdif(size(u, 2), 1);
coll = struct('ndim', size(u, 1), 'nmesh', size(u, 2), 'Dt', -D*2*pi);
end

function res = collocation(f, u, p, T, coll)
% Matrix of derivatives along the orbit
res = zeros(size(u));
D = reshape(u, [coll.ndim, coll.nmesh])*coll.Dt;
ii = 1:coll.ndim;
for i = 1:coll.nmesh
% Subtract the desired derivative from the actual derivative
res(ii) = D(ii) - T*f(T*(i-1)/coll.nmesh, u(ii), p)';
ii = ii + coll.ndim;
end
end``````