Abstract. We investigate rational maps with period-one and period-two cluster cycles. Given the definition of a cluster, we show that, in the case where the degree is d and the cluster is fixed, the Thurston class of a rational map is fixed by the combinatorial rotation number ρ and the critical displacement δ of the cluster cycle. The same result will also be proved in the case where the rational map is quadratic and has a period-two cluster cycle, and we will also show that the statement is no longer true in the higher-degree case.
IntroductionThe emergence of complex dynamics as a popular subject for mathematical research came about as a result of the rediscovery of the early twentieth-century works of Fatou [5,6] and Julia [7]. After a comparatively quiet period, the subject was given new life in the 1980s. Perhaps the most notable contributions were supplied by Douady and Hubbard, whose 'Orsay lecture notes' [1, 2] provide a number of enlightening and amazing results about the behaviour of such systems. Since then, the study of complex dynamical systems has grown enormously and is now a very fruitful area for research.In dynamical systems, one often wants to be able to understand the systems one works with up to some form of equivalence. The study of complex dynamics on the Riemann sphere is no different, and we are fortunate that there is a powerful criterion, due to Thurston, that tells us whether or not two rational maps on the sphere are equivalent. However, despite its simplicity and power, the criterion suffers from the fact that it can, in practice, be very difficult to check. In this paper, we investigate a specific class of maps, those bicritical rational maps with periodic cluster cycles of period one or period two (the latter only in the quadratic case), and show that in these cases the application of the Thurston criterion is simple and allows a classification of such maps. This work fits into the branch of complex dynamics which attempts to describe rational maps combinatorially (or perhaps symbolically), following a programme initiated by Douady, Hubbard and Thurston. Perhaps the most remarkable feature of the results in this paper is that the rational maps in question are classified purely by the behaviour within their clusters. It is hoped that the techniques in this paper can be refined to tackle a larger class of rational maps, perhaps