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Abstract We give examples of birational selfmaps of , whose dynamical degree is a transcendental number. This contradicts a conjecture by Bellon and Viallet. The proof uses a combination of techniques from algebraic dynamics and diophantine approximation. 

Let \begin{document}$$ f_c(z) = z^2+c $$\end{document} for \begin{document}$$ c \in {\mathbb C} $$\end{document}. We show there exists a uniform upper bound on the number of points in \begin{document}$$ {\mathbb P}^1( {\mathbb C}) $$\end{document} that can be preperiodic for both \begin{document}$$ f_{c_1} $$\end{document} and \begin{document}$$ f_{c_2} $$\end{document}, for any pair \begin{document}$$ c_1\not = c_2 $$\end{document} in \begin{document}$$ {\mathbb C} $$\end{document}. The proof combines arithmetic ingredients with complex-analytic: we estimate an adelic energy pairing when the parameters lie in \begin{document}$$ \overline{\mathbb{Q}} $$\end{document}, building on the quantitative arithmetic equidistribution theorem of Favre and Rivera-Letelier, and we use distortion theorems in complex analysis to control the size of the intersection of distinct Julia sets. The proofs are effective, and we provide explicit constants for each of the results. 

In this paper, we make partial progress on a function field version of the dynamical uniform boundedness conjecture for certain one-dimensional families $${\mathcal{F}}$$ of polynomial maps, such as the family $$f_{c}(x)=x^{m}+c$$ , where $$m\geq 2$$ . We do this by making use of the dynatomic modular curves $$Y_{1}(n)$$ (respectively $$Y_{0}(n)$$ ) which parametrize maps $$f$$ in $${\mathcal{F}}$$ together with a point (respectively orbit) of period $$n$$ for $$f$$ . The key point in our strategy is to study the set of primes $$p$$ for which the reduction of $$Y_{1}(n)$$ modulo $$p$$ fails to be smooth or irreducible. Morton gave an algorithm to construct, for each $$n$$ , a discriminant $$D_{n}$$ whose list of prime factors contains all the primes of bad reduction for $$Y_{1}(n)$$ . In this paper, we refine and strengthen Morton’s results. Specifically, we exhibit two criteria on a prime $$p$$ dividing $$D_{n}$$ : one guarantees that $$p$$ is in fact a prime of bad reduction for $$Y_{1}(n)$$ , yet this same criterion implies that $$Y_{0}(n)$$ is geometrically irreducible. The other guarantees that the reduction of $$Y_{1}(n)$$ modulo $$p$$ is actually smooth. As an application of the second criterion, we extend results of Morton, Flynn, Poonen, Schaefer, and Stoll by giving new examples of good reduction of $$Y_{1}(n)$$ for several primes dividing $$D_{n}$$ when $n=7,8,11$ , and $$f_{c}(x)=x^{2}+c$$ . The proofs involve a blend of arithmetic and complex dynamics, reduction theory for curves, ramification theory, and the combinatorics of the Mandelbrot set.