# Exact real arithmetic

## Introduction

Exact real arithmetic is an interesting area: it is based on a deep connection between

- numeric methods
- and deep theoretic foundations of algorithms (and mathematics).

Its topic: computable real numbers raise a lot of interesting questions rooted in mathematical analysis, arithmetic, but also Computability theory (see numbers-as-programs approaches).

Computable reals can be achieved by many approaches -- it is not one single theory.

### What it is *not*

Exact real arithmetic is not the same as *fixed* arbitrary precision reals (see `Precision(n)`

of Yacas).

Exact reals must allow us to run a huge series of computations, prescribing only the precision of the end result. Intermediate computations, and determining their necessary precision must be achieved automatically, dynamically.

Maybe another problem, but it was that lead me to think about exact real arithmetic: using some Mandelbrot-plotting programs, the number of iterations must be prescribed by the user at the beginning. And when we zoom too deep into these Mandelbrot worlds, it will become ragged or smooth. Maybe solving this particular problem does not necessarily need the concept of exact real arithmetic, but it was the first time I began to think about such problems.

See other numeric algorithms at Libraries and tools/Mathematics.

### Why are there reals at all which are defined exactly, but are not computable?

See e.g. Chaitin's construction.

## Theory

- Jean Vuillemin's Exact real computer arithmetic with continued fractions is very good article on the topic itself. It can also serve as a good introductory article, because it presents the connections to both mathematical analysis and Computability theory. It discusses several methods, and it describes some of them in more details.

- Martín Escardó's project A Calculator for Exact Real Number Computation -- its chosen functional language is Haskell, mainly because of its purity, lazyness, presence of lazy lists, pattern matching. Martín Escardó has many exact real arithetic materials also among his many papers.

- Jerzy Karczmarczuk's
*(wonderful, wonderful! gem of a)*paper with the funny title The Most Unreliable Technique in the World to compute pi describes how to compute Pi as a lazy list of digits.

## Implementations

See Libraries and tools/Mathematics

## Portal-like homepages

- Exact Computation: There are functional programming materials too, even with downloadable Haskell source.