dani2442 / bifurcationjax

Bifurcation library

BifurcationJax

This Jax package aims at performing automatic bifurcation analysis of finite-based dimensional equations F(u, λ)=0 where λ is real by taking advantage of iterative methods. Using JAX, a high-performance numerical computing tool, we are able to leverage the computing capacity and perform complex tasks in a matter of milliseconds. The package is inspired by the well-supported and holistic BiurcationKit package developed in the Julia programming language.

The following list enumerates the current functionalities of this package and the ones that will be incorporated in the following months:

• Nonlinear equations (Newton)
• Bifurcations local (equilibria) and global (periodic orbits)
• Branch Switching
• Bifurcation Diagram
• Continuation Methods
  • Predictor
    • Tangent Predictor
    • Secant Predictor
    • Natural Predictor
    • BorderedPredictor
  • Corrector
    • Newton Corrector
    • Mixed Corrector
    • PLAC
    • Moore-Penrose

Example 1

Let us study the equation $$\mathfrak{F}(x, \mu):= \mu + x - \frac{1}{3}x^3$$

def F(x, mu):
    return mu + x - jnp.pow(x, 3)/3

def plot_fn(p):
    return p.z[0]

p0 = 0.
x0 = jnp.array([-2.])

prob = BifurcationProblem(F, x0, p0,)
par = ContinuationPar(p_min=-1., p_max=1., dsmax=0.1, max_steps=500)
correction_params = CorrectorParams(method='PALC', epsilon=1e-3)
prediction_params = PredictorParams(method='tangent', k=0)
diagram = continuation(prob, prediction_params, correction_params, par, max_depth=1)

plot_bifurcation_diagram(diagram, plot_fn=plot_fn)
plt.show()

Example 2

Here is another example, this time we will study the equation

$$\mathfrak{F}(u, p)= x' = -x - y \quad y' = -pz + ry + s z^2 - y z^2 \quad z' = -q(x + z)$$

def maasch_rule(u, p):
    x, y, z = u[...,0], u[...,1], u[...,2]
    q, r, s, = 1.2, 0.8, 0.8
    dx = -x - y
    dy = -p*z + r*y + s*z*z - z*z*y
    dz = -q*(x + z)
    return jnp.stack([dx, dy, dz], axis=-1)


def plot_fn(p):
    return p.z[0]

p0 = 0.0
x0 = jnp.array([-1.4, -1.4, -1.4])

prob = BifurcationProblem(maasch_rule, x0, p0,)
par = ContinuationPar(p_min=-0.1, p_max=2., dsmax=0.05, max_steps=500)
prediction_params = PredictorParams(method='tangent', k=1)
correction_params = CorrectorParams(method='PALC', epsilon=1e-4)
branches = continuation(prob, prediction_params, correction_params, par, max_depth=1, k_start=0)


plot_bifurcation_diagram(branches, plot_fn=plot_fn)
plt.show()

Example 3

This time we will study the equation $$-u'' = \lambda u - au^3\quad\quad \text{ at }(0,1)$$ And $x(0)=x(1) = 0$.

N = 40
h = 1/N
t = jnp.linspace(0,1,N)
a=1

def F(x, p):
    x = jax.lax.dynamic_update_slice(jnp.zeros((N,)), x, (1,))
    u_xx = (x[2:] + x[:-2] - 2*x[1:-1])/(h**2)
    return u_xx + p*x[1:-1] - a*jnp.power(x[1:-1],3)
    
def plot_fn(p):
    if p.z[0]>0:
        return jnp.max(p.z[:-1])
    else:
        return jnp.min(p.z[:-1])

p0 = 0.
x0 = jnp.zeros((N-2,)) 

prob = BifurcationProblem(F, x0, p0,)
par = ContinuationPar(p_min=-5., p_max=200., dsmax=0.25, max_steps=1000, branch_switch='normal_orthogonal_direction')
correction_params = CorrectorParams(method='PALC', epsilon=1e-3)
prediction_params = PredictorParams(method='tangent', k=N-2)
branches = continuation(prob, prediction_params, correction_params, par, max_depth=2)


plot_bifurcation_diagram(branches, plot_fn=plot_fn)
plt.show()

Contribute

The package is under development and numourous functionalities will be incorporated in the following months