# Bifurcation Diagrams

Bifurcation diagrams can be produced for models generated by Catalyst through the use of the BifurcationKit.jl package. This tutorial gives a simple example of how to create such a bifurcation diagram.

Catalyst 13.0 and up require at least BifurcationKit v0.2.4.

First, we declare our reaction model. For this example we will use a bistable switch, but one which also contains a Hopf bifurcation.

```
using Catalyst
rn = @reaction_network begin
(v0 + v*(S * X)^n / ((S*X)^n + (D*A)^n + K^n), d), ∅ ↔ X
(X/τ, 1/τ), ∅ ↔ A
end
```

\[ \begin{align*} \varnothing &\xrightleftharpoons[d]{v0 + \frac{v \left( S X \right)^{n}}{\left( S X \right)^{n} + K^{n} + \left( A D \right)^{n}}} \mathrm{X} \\ \varnothing &\xrightleftharpoons[\frac{1}{\tau}]{\frac{X}{\tau}} \mathrm{A} \end{align*} \]

Next, we specify the system parameters for which we wish to plot the bifurcation diagram. We also set the parameter we wish to vary in our bifurcation diagram, as well as the interval to vary it over. Finally, we set which variable we wish to plot the steady state values of in the bifurcation plot.

```
p = Dict(:S => 1., :D => 9., :τ => 1000., :v0 => 0.01,
:v => 2., :K => 20., :n => 3, :d => 0.05)
bif_par = :S # bifurcation parameter
p_span = (0.1, 20.) # interval to vary S over
plot_var = :X # we will plot X vs S
```

When creating a bifurcation diagram, we typically start at some point in parameter-space. We will simply select the beginning of the interval over which we wish to compute the bifurcation diagram, `p_span[1]`

. We thus create a modified parameter set where `S = 0.1`

. For this parameter set, we also guess the steady state of the system. While a good estimate could be provided through an ODE simulation, BifurcationKit does not require the guess to be very accurate.

```
p_bstart = copy(p)
p_bstart[bif_par] = p_span[1]
u0 = [:X => 1.0, :A => 1.0]
```

Finally, we extract the ODE derivative function and its jacobian in a form that BifurcationKit can use:

```
oprob = ODEProblem(rn, u0, (0.0, 0.0), p_bstart; jac = true)
F = (u,p) -> oprob.f(u, p, 0)
J = (u,p) -> oprob.f.jac(u, p, 0)
```

In creating an `ODEProblem`

an ordering is chosen for the initial condition and parameters, and regular `Float64`

vectors of their numerical values are created as `oprob.u0`

and `oprob.p`

respectively. BifurcationKit needs to know the index in `oprob.p`

of our bifurcation parameter, `:S`

, and the index in `oprob.u0`

of the variable we wish to plot, `:X`

. We calculate these as

```
# get S and X as symbolic variables
@unpack S, X = rn
# find their indices in oprob.p and oprob.u0 respectively
bif_idx = findfirst(isequal(S), parameters(rn))
plot_idx = findfirst(isequal(X), species(rn))
```

Now, we load the required packages to create and plot the bifurcation diagram. We also bundle the information we have compiled so far into a `BifurcationProblem`

.

```
using BifurcationKit, Plots, LinearAlgebra, Setfield
bprob = BifurcationProblem(F, oprob.u0, oprob.p, (@lens _[bif_idx]);
record_from_solution = (x, p) -> x[plot_idx], J = J)
```

`WARNING: using Plots.wrap in module BifurcationKit conflicts with an existing identifier.`

Next, we need to specify the input options for the pseudo-arclength continuation method (PACM) which produces the diagram.

```
bopts = ContinuationPar(dsmax = 0.05, # Max arclength in PACM.
dsmin = 1e-4, # Min arclength in PACM.
ds = 0.001, # Initial (positive) arclength in PACM.
max_steps = 100000, # Max number of steps.
p_min = p_span[1], # Min p-val (if hit, the method stops).
p_max = p_span[2], # Max p-val (if hit, the method stops).
detect_bifurcation = 3) # Value in {0,1,2,3}
```

Here `detectBifurcation`

determines to what extent bifurcation points are detected and how accurately their values are determined. Three indicates to use the most accurate method for calculating their values.

We are now ready to compute the bifurcation diagram:

`bf = bifurcationdiagram(bprob, PALC(), 2, (args...) -> bopts)`

Finally, we can plot it:

`plot(bf; xlabel = string(bif_par), ylabel = string(plot_var))`

Here, the Hopf bifurcation is marked with a red dot and the fold bifurcations with blue dots. The region with a thinner line width corresponds to unstable steady states.

This tutorial demonstrated how to make a simple bifurcation diagram where all branches are connected. However, BifurcationKit.jl is a very powerful package capable of a lot more. For more details, please see that package's documentation: BifurcationKit.jl.

## Citation

If you use this functionality in your research, please cite the following paper to support the author of the BifurcationKit package:

```
@misc{veltz:hal-02902346,
title = {{BifurcationKit.jl}},
author = {Veltz, Romain},
url = {https://hal.archives-ouvertes.fr/hal-02902346},
institution = {{Inria Sophia-Antipolis}},
year = {2020},
month = Jul,
keywords = {pseudo-arclength-continuation ; periodic-orbits ; floquet ; gpu ; bifurcation-diagram ; deflation ; newton-krylov},
pdf = {https://hal.archives-ouvertes.fr/hal-02902346/file/354c9fb0d148262405609eed2cb7927818706f1f.tar.gz},
hal_id = {hal-02902346},
hal_version = {v1},
}
```