Let \( \phi(t)\in \mathbb Q(t)\) be of degree \( d\geq 2\). For a given rational number \( x_0\) , define \( x_{n+1} = \phi(x_n)\) for each \( n\geq 0\). If this sequence is not eventually periodic, and if \( \phi\) does not
lie in one of two explicitly determined affine conjugacy classes of rational functions, then the numerator of
\( x_{n+1}-x_n\) has a primitive prime factor for all sufficiently large \( n\). The
same result holds for the exceptional maps provided that one looks for primitive prime factors
in the denominator of \( x_{n+1}-x_n\) . Hence the result for each rational function \( \phi\) implies (a new proof) that there are infinitely many primes.
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