Let
ϕ(t)∈Q(t) be of degree
d≥2. For a given rational number
x0 , define
xn+1=ϕ(xn) for each
n≥0. If this sequence is not eventually periodic, and if
ϕ does not
lie in one of two explicitly determined affine conjugacy classes of rational functions, then the numerator of
xn+1−xn 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
xn+1−xn . Hence the result for each rational function
ϕ implies (a new proof) that there are infinitely many primes.
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