Floor of Simple Finite Continued Fraction

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Theorem

Let $\sequence {a_k}_{k \mathop \ge 0}$ be a simple finite continued fraction of length $n \ge 0$.

Let $x = [a_0, \ldots, a_n]$ be its value.


Then the floor of $x$ is the partial denominator $a_0$:

$\floor x = a_0$

unless $n = 1$ and $a_1 = 1$, in which case $x = \floor x = a_0 + 1$.


Proof

Length 0

If $n = 0$, we have $x = \sqbrk {a_0, \ldots, a_n} = a_0$ by definition of value.

By Real Number is Integer iff equals Floor:

$\floor x = \floor {a_0} = a_0$


Length 1

If $n = 1$, then:

$x = \sqbrk {a_0, a_1} = a_0 + \dfrac 1 {a_1}$

If $a_1 > 1$, then:

$a_0 < a_0 + \dfrac 1 {a_1} < a_0 + 1$

By definition of floor function:

$\floor x = a_0$

If $a_1 = 1$, then $x = a_0 + 1$.

By Real Number is Integer iff equals Floor:

$\floor x = a_0 + 1$


Length at least 2

By Value of Finite Continued Fraction of Real Numbers is at Least First Term:

$\sqbrk {a_0, \ldots, a_n} > a_0$
$\sqbrk {a_1, \ldots, a_n} > a_1 \ge 1$

Thus:

$a_0 < \sqbrk {a_0, \ldots, a_n} = a_0 + \dfrac 1 {\sqbrk {a_1, \ldots, a_n} } < a_0 + 1$

By definition of floor function:

$\floor x = a_0$

$\blacksquare$


Also see