# Accuracy of Convergents of Continued Fraction

## Theorem

### Accuracy of Convergents of Continued Fraction Expansion of Irrational Number

Let $x$ be an irrational number.

Let $(a_0, a_1, \ldots)$ be its continued fraction expansion.

Let $\left \langle {C_n}\right \rangle_{n \geq 0}$ be its sequence of convergents.

Let $p_0, p_1, p_2, \ldots$ and $q_0, q_1, q_2, \ldots$ be its numerators and denominators.

Then:

$\forall k \ge 1: \left\vert{x - \dfrac {p_{k + 1} } {q_{k + 1} } }\right\vert < \dfrac 1 {q_{k + 1} q_{k + 2} } \le \dfrac 1 {2 q_k q_{k + 1} } < \left\vert{x - \dfrac {p_k} {q_k} }\right\vert$

Thus:

The left hand side of the inequality gives an indication of how close each convergent gets to its true value.
The right hand side gives a bound that limits its accuracy.

### Accuracy of Convergents of Convergent Simple Infinite Continued Fraction

Let $C = \tuple {a_0, a_1, \ldots}$ be an simple infinite continued fraction in $\R$.

Let $C$ converge to $x \in \R$.

For $n \ge 0$, let $C_n = \dfrac {p_n} {q_n}$ be the $n$th convergent of $C$, where $p_n$ and $q_n$ are the $n$th numerator and denominator.

Then for all $n \ge 0$:

$\size {x - \dfrac {p_n} {q_n} } < \dfrac 1 {q_n q_{n + 1} }$

## Caution

We have a posteriori, by Correspondence between Irrational Numbers and Simple Infinite Continued Fractions, that the statements above coincide, or at least partially.

The point is that they are used when proving the correspondence.