Rational approximations to a square root p k can be produced by iterating the transformation f(x) = (dx+k)/(x+d) starting from 1 for any d 2 N. We show that these approximations coincide infinitely often with continued fraction convergents if and only if 4d2/(k d2) is an integer, in which case the continued fraction has a rich structure. It consists of the concatenation of the continued fractions of certain explicitly describable rational numbers, and it belongs to one of infinitely many families of continued fractions whose terms vary linearly in two parameters. We also give conditions under which the orbit {f n(1)} consists exclusively of convergents or semiconvergents and prove that with few exceptions it includes all solutions p/q to the Pell equation p2 kq2 = ±1.
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