# Exact real arithmetic

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## 1 Introduction

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

• numeric methods
• and deep theoretic fondations 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.

### 1.1 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 on 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 need necessarily the concept of exact real arithmetic, but it was the first time I began to think on such problems.

See other numeric algorithms at Libraries and tools/Mathematics.

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

See e.g. Chaitin's construction.

## 3 Portal-like homepages

### 3.1 Exact Computation

There are functional programming materials too, even with downloadable Haskell source.

### 3.2 ExactRealArithmetic

This HaWiki article provides links to many implementations.