🟢 pp2 example from AUTO07p (aBD + Hopf aBS)
The goal of this example is to show how to use automatic bifurcation diagram computation for a simple ODE.
The following equations are a model of type predator-prey. The example is taken from Auto07p:
\[\begin{array}{l} u_{1}^{\prime}=3 u_{1}\left(1-u_{1}\right)-u_{1} u_{2}-p_1\left(1-e^{-5 u_{1}}\right) \\ u_{2}^{\prime}=-u_{2}+3 u_{1} u_{2} \end{array}\]
It is easy to encode the ODE as follows
using Revise, Plots
using BifurcationKit
# function to record information from a solution
recordFromSolution(x, p) = (u1 = x[1], u2 = x[2])
function pp2!(dz, z, p, t = 0)
(;p1, p2, p3, p4) = p
u1, u2 = z
dz[1] = p2 * u1 * (1 - u1) - u1 * u2 - p1 * (1 - exp(-p3 * u1))
dz[2] = -u2 + p4 * u1 * u2
dz
end
# parameters of the model
par_pp2 = (p1 = 1., p2 = 3., p3 = 5., p4 = 3.)
# initial condition
z0 = zeros(2)
# bifurcation problem
prob = BifurcationProblem(pp2!, z0, par_pp2,
# specify the continuation parameter
(@lens _.p1), record_from_solution = recordFromSolution)
Automatic bifurcation diagram computation
We set up the options or the continuation
# continuation options
opts_br = ContinuationPar(p_min = 0.1, p_max = 1.0, dsmax = 0.01,
# number of eigenvalues
nev = 2,
# maximum number of continuation steps
max_steps = 1000,)
We are now ready to compute the diagram
diagram = bifurcationdiagram(prob, PALC(),
# very important parameter. It specifies the maximum amount of recursion
# when computing the bifurcation diagram. It means we allow computing branches of branches of branches
# at most in the present case.
3,
ContinuationPar(opts_br; ds = -0.001, dsmax = 0.01, n_inversion = 8, detect_bifurcation = 3)
)
scene = plot(diagram; code = (), title="$(size(diagram)) branches", legend = false)
Branch of periodic orbits with collocation method
As you can see on the diagram, there is a Hopf bifurcation indicated by a red dot. Let us compute the periodic orbit branching from the Hopf point.
We first find the branch
# branch of the diagram with Hopf point
brH = get_branch(diagram, (2,1)).γ
# continuation parameters
opts_po_cont = ContinuationPar(dsmax = 0.1, ds= 0.0001, dsmin = 1e-4,
tol_stability = 1e-4)
br_po = continuation(
brH, 1, opts_po_cont,
PeriodicOrbitOCollProblem(20, 4);
# plot = true, verbosity = 2,
record_from_solution = (x, p) -> begin
xtt = get_periodic_orbit(p.prob, x, p.p)
return (max = maximum(xtt[1,:]),
min = minimum(xtt[1,:]),
period = x[end])
end,
plot_solution = (x,p;k...) -> begin
xtt = get_periodic_orbit(p.prob, x, p.p)
plot!(xtt.t, xtt[1,:]; k..., marker=:d, markersize=1, label = "")
end,
finalise_solution = (z, tau, step, contResult; prob = nothing, kwargs...) -> begin
# limit the period
getperiod(prob, z.u, nothing) < 50
end,
normC = norminf)
plot(diagram); plot!(br_po, branchlabel = "Periodic orbits", legend = :bottomright)
Let us now plot an orbit
# extract the different components
orbit = get_periodic_orbit(br_po, 30)
plot(orbit.t, orbit[1,:]; label = "u1", markersize = 2)
plot!(orbit.t, orbit[2,:]; label = "u2", xlabel = "time", title = "period = $(orbit.t[end])")