Rational approximation

Introduction

The main theme of this article is the question how well a given real number $x$ can be approximated by rational numbers. Of course, since the rationals are dense on the real line, we, surely, can make the difference between $x$ and its rational approximation $\frac pq$ as small as we wish. The problem is that, as we try to make $\frac pq$ closer and closer to $x$ , we may have to use larger and larger $p$ and $q$ . So, the reasonable question to ask here is how well can we approximate $x$ by rationals with not too large denominators.

Trivial theorem

Every real number $x$ can be approximated by a rational number $pq$ with a given denominator $q\ge 1$ with an error not exceeding $\frac 1{2q}$ .

Proof

Note that the closed interval $\left[qx-\frac12,qx+\frac12\right]$ has length $1$ and, therefore, contains at least one integer. Choosing $p$ to be that integer, we immediately get the result.

So, the interesting question is whether we can get a smaller error of approximation than $\frac 1q$ . Surprisingly enough, it is possible, if not for all $q$ , then, at least for some of them.

Dirichlet's theorem

Let $n\ge 1$ be any integer. Then there exists a rational number $\frac pq$ such that $1\le q\le n$ and $\left|x-\frac pq\right|<\frac 1{nq}$ .

Proof of Dirichlet's theorem

Consider the fractional parts $\{1\cdot x\}, \{2\cdot x\},\dots, \{n\cdot x\}$ . They all belong to the half-open interval $[0,1)$ . Represent the interval $[0,1)$ as the union of $n$ subintervals $[0,1)=\left[0,\frac 1n\right)\cup\dots\cup\left[\frac{n-1}{n},1\right),$ .

to be continued

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