# Continued fraction and classification of real numbers.

I would be grateful if anyone can tell if there are any methods to classify real numbers using continued fraction. eg: Suppose $[a_0;a_1,a_2,\ldots,a_n]$ is the representation of some real number $\alpha$ then

1) if $n$ is finite then it represents a rational number.

2) if $n$ is infinite then it is an irrational number.

3) if $\alpha$ is a quadratic irrational then the pattern repeats indefinitely with a period of some number $t$. What we can say about the converse?

4) for 2) case how we can distinguish between algebraic and transcendental numbers?

• This is all standard stuff. You're on to something. Have a look around for the continued fraction algorithm; in particular, look for a proof that it works. That'll answer most (if not all) of your questions. – Shaun Jun 18 '14 at 12:40

If $n$ is finite:

$n\in\mathbb{Q}$

This is because a finite continued fraction is the result of a finite amount of additions and divisions of integers.

If $n$ is infinite and periodic

$n$ is an irrational root of $ax^2+bx+c=0$ where $a,b,c\in\mathbb{Q}$

If $n$ is infinite and non-periodic

$n$ is not expressible as a root of a second degree or lower polynomial.

Algebraic vs transcendental do not show any differences as far as I figured out.

Transcendental numbers can show patterns, for example:

$$e=[2;1,2,1,1,4,1,1,6,1,1,8,1,1,10,\ldots]$$

• So you are saying that converse of 3) holds? – schzan Jun 19 '14 at 14:12
• @shzan It does. – Alice Ryhl Jun 20 '14 at 6:04

I believe there is no presently known "reasonably explicit" description (i.e. an $n$-term recursion formulation for some positive integer $n)$ of continued fraction expansions that can be used to characterize even just the cubic irrationalities, although a recent paper by Nadir Murru---On the Hermite problem for cubic irrationalities---appears to give such a description for cubic irrationalities when multidimensional continued fraction expansions are used.

I've known about this for maybe a year, but I have not yet looked into what this all means, so hopefully an expert can provide more details and correct any inaccuracies I may have made.

• Thanks for mentioning the paper. – schzan Jun 19 '14 at 14:19