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Quadratic Irrationals

Definition 1.1   An element $ \alpha\in\mathbb{R}$ is a quadratic irrational if it is irrational and satisfies a quadratic polynomial.

Thus, e.g., $ (1+\sqrt{5})/2$ is a quadratic irrational. Recall that

$\displaystyle \frac{1+\sqrt{5}}{2} = [1,1,1,\ldots].
$

The continued fraction of $ \sqrt{2}$ is $ [1,2,2,2,2,2,\ldots]$, and the continued fraction of $ \sqrt{389}$ is

$\displaystyle [19,1,2,1, 1, 1, 1, 2, 1, 38, 1, 2, 1, 1, 1, 1, 2, 1, 38,\ldots].$

Does the $ [1,2,1, 1, 1, 1, 2, 1, 38]$ pattern repeat over and over again??



William A Stein 2001-10-25