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# Arithmetical Dynamics: Part 4

We are here with the Part 4 of the Arithmetical Dynamics Series. Let's get started....

Arithmetical dynamics is the combination of dynamical systems and number theory in mathematics.

$P^m(z) = z \ and \ P^N(z)=z \ where \ m|N \Rightarrow (P^m(z) - z) | (P^N(z)-z)$

#### The proof of the theorem in Part 0 :

Let , P be the polynomials satisfying the hypothesis of theorem 6.2.1 .

Let , $K = \{ z \in C | P^N(z) =z \} \\$

and let $M =\{ m \in Z : 1 \leq m \leq N , m|N \} \\$

then each $z \in K$ is a fixed point of $P^m$ for some $m \in M$ and we let m(z) be the minimal such m . $\\$

The proof depends on establishing the inequalities , $$d^{N-1} (d-1) \leq \sum_{k} [\mu (N, z) - \mu (m(z) ,z) ] \\ \leq N(d-1)$$ ......................................$(1)$ where $\mu (n, w)$ is the no of fixed points of $P^n$ at w .$$(1) \Rightarrow d^{N-1} \leq N \\ therefore , N= 1 + (N-1) \leq 1 + (N - 1)(d - 1) \leq [1 + (d-1)]^{N-1} = d^{N - 1} \leq N$$

Make sure you visit the Arithmetical Dynamics Part 3 post of this Series before the Arithmetical Dynamics Part 4.

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