# Ivory: A Mammoth Numerical Tower

Ivory is a “numerical tower”: a structured approach to implementing numbers and arithmetic on a computer, which makes them amenable to programming in a rather simple way. Compared to other numerical towers, Ivory removes a few levels and adds many more. Whilst designed for computer programmers, these explanations and descriptions may also be of use to non-programmers (feel free to skip the implementation details!).

## Goals

Ivory aims to be more than yet another library of standalone functions/classes. It instead takes a holistic view about incorporating useful mathematics more deeply into a language. Its design is inspired by the numerical system of Scheme, and uses the language extension mechanisms of Racket to support a wider variety of notation.

Ivory’s focus is on exactness and correctness, since it is hard to build software on unstable foundations. This precludes support for approximate formats like IEEE754 floats, unums, posits, etc. (we can still convert to such formats, but they are not in the tower; just like, say, strings are not in the tower but we can convert to them).

As a numerical tower, Ivory is focused on the representation of numbers, and designing a one-dimensional nesting of those representations, as seen in the figure above. The main design principle for choosing what to put in the tower is the existence of a unique normal form for each number, such that two numbers are only equal when their representations are identical. This rules out very general representations, like closed-form expressions, power series and computable numbers, since their equality is undecidable. This requirement for exact, unique representations is why Ivory does not currently include transcendental numbers like τ and 𝑒, or operations like sin and log.

The levels `real`, `number` and `expression` provide no extra values or operations, and act merely as alternative names for the levels above them (`algebraic`, `geometric` and `algebraic-expression`, respectively). They mark important transitions in the tower, below which some important results can no longer be assumed to hold. For example, values above the `real` level can be totally ordered into a “number line”, whilst such comparisons don’t generally hold for the levels below. Applications which need comparable numbers should check if they’re `real`, rather than `algebraic`; that way, any extra levels inserted between these in future revisions will be automatically supported.

## Required Knowledge

These pages do not assume any knowledge of advanced mathematics, but do assume some familiarity with high-school algebra, like how to multiply groups of terms, e.g.

(2+a)(5+3b) = 10 + 5a + 6b + 3ab

We will be writing a lot of s-expressions (AKA Lisp syntax or prefix notation) since that matches the Scheme programming language used by Ivory’s reference implementation. This style may be unfamiliar, but is pretty simple and has the benefit of removing ambiguities like precedence (which tend to plague those trying to learn mathematics!).

In an s-expression, operations are written `(in parentheses)`, with the operation’s name/symbol first and its inputs after. For example, the above equation could be written as an s-expression like this :

At times we’ll be using Scheme’s `quote` and `quasiquote` features, which may need a little explanation for those who aren’t used to Scheme programming. A “quotation” is prefixed by a single-quote/apostrophe `'`, and is used to represent data without attempting to execute it. For example, the value of `'(+ 2 5)` is a list containing three elements (like `["+", 2, 5]` in other languages; and unlike `(+ 2 5)`, whose value is the `number` `7`). A “quasiquotation” is prefixed by a backtick ```, and is like a quotation except that expressions prefixed with a comma `,` are “unquoted”; for instance ``(10 plus 20 is ,(+ 10 20))` gives the list `(10 plus 20 is 30)` (quasiquoting is similar to “string splicing” in other languages, like `"10 plus 20 is \${10 + 20}"`).

## Implementation

This project is split across several pages, which describe the tower’s design, the basic mathematics and applications of its components, as well as implementing them in the Racket programming language. The most interesting parts are shown in a literate programming style using active code. The full Racket code is always linked at the bottom of each page; alongside a “view source” link to the page’s Markdown.

The following pages give introductory information, without defining any code:

• Numerical Towers introduces the idea, its tradeoffs, and contrasts it with other approaches to implementing arithmetic.
• Numbers In Scheme And Racket describes the numerical towers defined by the Scheme standards and the Racket language’s standard library, which Ivory builds on.

These pages introduce some of the mathematical ideas that guide our design, and implement useful definitions that make the subsequent sections easier:

• Sums And Products implements these operations symbolically, explores the resulting tree structures, and defines algorithms to normalise them. Many of our tower’s levels are based on these trees.
• By Your Powers Combined extends the sums and products representation to include exponents, useful for reciprocals and roots.
• Negatives And Inverses are used to implement subtraction and division in terms of addition and multiplication, which simplifies our arithmetic.

The levels of the Ivory tower are explained and implemented in the following:

• Zero, One, Many describes the top levels of the tower.
• Manifest Decimation describes `decimal`, those fractions with a power of ten as denominator (i.e. with a finite number of non-repeating decimal places). Also includes the binary equivalent, known as `dyadic`, which are well-suited to efficient algorithms.
• Radicals implements the `algebraic` level, representing roots symbolically.
• Geometric Units begins the definition of `geometric`, by introducing symbolic values separate from the usual number line.
• Complex And Hypercomplex Numbers use the geometric units to construct mezzanine levels inside `geometric`.
• Geometric Algebra completes the implementation of the `geometric` level, with multi-vectors and graded operations.
• Secure, Contain, Project uses `geometric` numbers to implement a library for projective geometry (points, lines, planes, etc.)
• With Jubilee And Circle Line extends the above library to the conformal geometry of circles, spheres, etc. where “flat” shapes like lines and planes are treated as having infinite radius.
• Indeterminates implements the `univariate-monomial`, level, and introduces the `monomial` and `homogeneous` mezzanines from `polynomial`.
• Polynomials finishes the implementation of the `polynomial` level and explores its `univariate` mezzanine.
• Expressions extends `polynomial` to form the `rational-expression` and `algebraic-expression` levels, which are the most general numerical values supported by Ivory.

### Code

The entire Ivory codebase, generated from these pages, is archived in the following link:

TODO