The rational part of a periodic continued fraction

Kurt Girstmair

Published 2017-07-11Version 1

Let $x$ be a periodic continued fraction with the initial block $0$ and the repeating block $c_1,\ldots,c_n$. So $x$ is a quadratic irrational of the form $x=a+\sqrt b$, where $a$, $b$ are rational numbers, $b>0$, $b$ not a square. The numbers $a$ and $\sqrt b$ are uniquely determined by $x$. In general it is difficult to say what the influence of a certain digit of the repeating block on the appearance of $x$ is. We highlight a noteworthy exception from this rule. Indeed, the magnitude of $2a$ is essentially determined by the last digit $c_n$ of the repeating block, the fractional part of $2a$, however, is independent of $c_n$. Of particular interest is the case $2a\in\mathbb Z$, which occurs if, and only if, the sequence $c_1,\ldots,c_{n-1}$ is symmetric.