# Normal form of the Bautin bifurcation

We follow the paper[Kuznetsov] and consider a Cauchy problem

$$$\dot x=\mathbf F(x,p).$$$

We denote by $\mathbf L$ the jacobian of $\mathbf F$ at the bifurcation point $(x_0,p_0)$. We choose a basis such that:

$$$\mathbf L q=i \omega_{0} q, \quad \mathbf L^{T} p=-i \omega_{0} p, \quad \langle p, q\rangle=1.$$$

Under some conditions, $x(t)\approx x_0+2\Re w(t)q$ where $w$ satisfies the normal form:

$$$\dot{w}=i \omega_{0} w+\frac{1}{2} G_{21} w|w|^{2}+\frac{1}{12} G_{32} w|w|^{4}+O\left(|w|^{6}\right).\tag{E}$$$

The second Lyapunov coefficient is

$$$l_2:=\frac{1}{12} \operatorname{Re} G_{32}.$$$

## Normal form computation

The normal form (E) can be automatically computed as follows

get_normal_form(br::ContResult, ind_bif::Int;
verbose = false,
ζs = nothing,
lens = getlens(br),
kwargs...)

br is a branch computed after a call to continuation with detection of bifurcation points enabled and ind_bif is the index of the bifurcation point on the branch br. The above call returns a point with information needed to compute the bifurcated branch. For more information about the optional parameters, we refer to get_normal_form. The result returns an object of type Bautin.

Note

You should not need to call get_normal_form except if you need the full information about the branch point.

## Predictor

The predictor for a non trivial guess at distance $\delta p$ from the bifurcation point is provided by the method

BifurcationKit.predictorMethod
predictor(gh, , ϵ; verbose, ampfactor)


Compute the predictor for the curve of Folds of periodic orbits near the Bautin bifurcation point.

Reference

Kuznetsov, Yu A., H. G. E. Meijer, W. Govaerts, and B. Sautois. “Switching to Nonhyperbolic Cycles from Codim 2 Bifurcations of Equilibria in ODEs.” Physica D: Nonlinear Phenomena 237, no. 23 (December 2008): 3061–68. https://doi.org/10.1016/j.physd.2008.06.006.

source

## References

• Kuznetsov

Kuznetsov, Yu. A. “Numerical Normalization Techniques for All Codim 2 Bifurcations of Equilibria in ODE’s.” SIAM Journal on Numerical Analysis 36, no. 4 (January 1, 1999): 1104–24. https://doi.org/10.1137/S0036142998335005.