### Period doubling initialization

The natural way to start a Period Doubling bifurcation curve continuation is to start it from a Period Doubling bifurcation point on a limit cycle curve. This can be done using the following command: `[x0,v0]=init_PD_PD(@odefile, x, s, ap, ntst, ncol)`. `x` should be the `x` as returned by the previous limit cycle continuation. `s` is the special point structure of the detected Period Doubling point on the limit cycle curve. `odefile` specifies the ode-file to be used. `ap` is the active parameter and `ntst` and `ncol` are again the number of mesh and collocation points for the discretization.

MATCONT provides four other initializers to start the continuation of PD curves from a codim2 bifurcation of limit cycles, namely `init_GPD_PD.m,` `init_LPPD_PD.m,` `init_PDNS_PD.m` and `init_R2_PD.m.` These initializers are added for ease of use; they only refer back to `init_PD_PD.m.`

We note that MATCONT also provides an initializer `init_PD_PD2.m` which is merely an alternative to `init_PD_PD.m`; it is not to be confused with the initializer `init_PD_LC.m` which initializes the computation of the period doubled curve from a PD point.