## Rational maps with a superattracting 2-cycle:

Quadratic rational maps f_{a}(z) = (z

^{2}+ a) / (z

^{2}- 1) form a family denoted by V

_{2}since the critical value ∞ is 2-periodic. The videos illustrate various applications of the Thurston algorithm to construct these maps from polynomials or from each other. The visualization of mating and anti-mating is based on the Buff–Chéritat approach to slow mating and tuning.

Most of the videos below give only six different maps, but up to four different constructions of the same map.

Here they are shown in parallel: play — show.

Left top: mating of z

^{2}+ i and z

^{2}- 1.

Right top: anti-mating of z

^{2}- q

^{2}and z

^{2}+ q with q

^{3}= √2 - 1.

Left bottom: repelling-preperiodic capture at the inverted basilica.

Right bottom: regluing of a map of type C.

### Mating and repelling-preperiodic capture

Two postcritically finite quadratic polynomials P(z) and Q(z) are combined to define the formal mating: in the lower and upper half-spheres, the new map is non-analytically conjugate to P(z) or Q(z), respectively. The (modified) Thurston algorithm gives a rational map, if P(z) and Q(z) do not belong to conjugate limbs of the Mandelbrot set. In particular, if Q(z) = z^{2}- 1 is the basilica polynomial and P(z) does not belong to the 1/2-limb, the resulting map will be of the form f

_{a}(z) . It is conjugate to the topological mating of P(z) and Q(z), where the filled Julia sets are glued together.

If P(z) is critically preperiodic, the same mating can be constructed by a preperiodic capture: in the dynamic plane of Q(z), the critical value ∞ is moved to a preperiodic point along some arc. The videos show this process inverted, i.e., the basilica contains ∞, and 0 is moved.

s3: the classical mating of rabbit and basilica, displayed on the sphere: play — show.

p4: primitive period 4: play — show.

12: preperiod 1, period 2. The repelling 2-periodic points are distinct in the formal mating, but identified in the limit and in the topological mating: play — show.

The same mating as a preperiodic capture. The identification is immediate: play — show.

14: preperiod 1, period 4. The formal mating has no removable obstruction: play — show.

And the same mating as a preperiodic capture: play — show.

32: preperiod 3, period 2. A shared basilica mating with 7/24 on the left hand side and 5/24 on the right hand side: play — show.

The locus of P in the 1/3-limb mated with the basilica: play — show.

Limb and mating locus together: play — show.

Deformation in parameter space from

*M*to V

_{2}: play — show.

### Anti-mating

The formal anti-mating was defined by Douady and Timorin corresponding to two polynomials acting between two planes, P(z) = z^{2}+ p from the first plane to the second plane, and Q(z) = z

^{2}+ q in the opposite direction. Consider these planes as half-spheres to describe the formal anti-mating. The topological and the slow anti-mating are obtained by gluing the filled Julia sets of Q°P and P°Q along their boundaries or along equipotential lines, respectively. If p = -q

^{2}, the critical point at ∞ will be 2-periodic, so the anti-mating belongs to V

_{2}. See corresponding regions of the connectedness locus of (z

^{2}- q

^{2})

^{2}+ q and of V

_{2}. While mating gives hyperbolic maps of type D only, anti-mating gives type C and B as well. (For the Multibrot family with p = 0, the anti-mating of z

^{2}and z

^{2}+ q is bitransitive.) Read more on the poster from the conference in honor of John H. Hubbard, Bremen 2015.

p4: anti-mating gives the primitive period 4 (see Julia sets): play — show.

12: anti-mating gives preperiod 1 and period 2, identified to 1 (see Julia sets): play — show.

c4: anti-mating gives a capture of preperiod 4 (see Julia sets): play — show.

### Hyperbolic capture and regluing

Starting with the basilica polynomial, move the critical value ∞ to a preimage of the 2-periodic critical point 0, preferably along external and internal rays. Again, this process will be shown inverted, with 0 moved to the basilica around ∞.Both hyperbolic capture and repelling-preperiodic capture can be applied to postcritcally finite maps f

_{a}(z) in V

_{2}, not only to the inverted basilica. In particular, for a C component the center and the preperiodic maps on the boundary can be related by moving the critical value f

_{a}(0); this corresponds to the topological process of regluing.

c1: the critical value 0 is moved to a preimage of ∞ of order 1. The Julia set is symmetric under reflection, quarter-rotation, and inversion: play — show.

c4: the critical value 0 is moved to a preimage of ∞ of order 4. Note that the resulting rational map was obtained as an anti-mating as well: play — show.

12 as a regluing of c1: play — show.

c1 as a regluing of 12: play — show.

14 as a regluing of c1: play — show.

c1 as a regluing of 14: play — show.