# Introduction

## Exact numbers in Calcium

The core idea behind Calcium is to represent real and complex numbers as elements of extension fields

\[\mathbb{Q}(a_1, \ldots, a_n)\]

of the rational numbers, where the extension numbers \(a_k\) are described by symbolic expressions (which may depend on other fields recursively). The system constructs such fields automatically as needed to represent the results of computations. Any extension field is isomorphic to a formal field

\[\mathbb{Q}(a_1,\ldots,a_n) \;\; \cong \;\; K_{\text{formal}} := \operatorname{Frac}(\mathbb{Q}[X_1,\ldots,X_n] / I)\]

where I is the ideal of algebraic relations among the extension numbers. The relations may involve algebraic numbers (for example: \(i^2 + 1 = 0\)), transcendental numbers (for example: \(e^{-\pi} \cdot e^{\pi} = 1\)), or combinations thereof.

Computation in the formal field depends (in general) on multivariate polynomial arithmetic together with use of a Gröbner basis for the ideal. The map from the formal field to the true complex field is maintained using arbitrary-precision ball arithmetic where necessary.

As an important special case, Calcium can be used for arithmetic in algebraic number fields (embedded explicitly in \(\mathbb{C}\))

\[\mathbb{Q}(a) \cong \mathbb{Q}[X] / \langle f(X) \rangle\]

with excellent performance thanks to internal use of the Antic library.

It will not always work perfectly: although Calcium by design should never give a mathematically erroneous answer, it may be unable to simplify a result as much as expected and it may be unable to decide a predicate (in which case it can return “Unknown”). Equality is at least decidable over the algebraic numbers \(\overline{\mathbb{Q}}\) (for practical degrees and bit sizes of the numbers!), and in certain cases involving transcendentals. We hope to improve Calcium’s capabilities gradually through enhancements to its built-in algorithms and through customization options.

### Usage details

To understand how Calcium works more concretely, see Calcium example programs and the documentation for the main Calcium number type (`ca_t`):

- ca.h – exact real and complex numbers

Implementation details for extension numbers and formal fields can be found in the documentation of the corresponding modules:

- ca_ext.h – real and complex extension numbers

- ca_field.h – extension fields

The following modules are used internally for arithmetic in transcendental number fields (rational function fields) \(\mathbb{Q}(x_1,\ldots,x_n)\) and over the field of algebraic numbers \(\overline{\mathbb{Q}}\), respectively. They may be of independent interest:

- fmpz_mpoly_q.h – multivariate rational functions over Q

- qqbar.h – algebraic numbers represented by minimal polynomials

## FAQ

Isn’t x = 0 undecidable?

In general, yes: equality over the reals is undecidable. In practice, much of calculus and elementary number theory can be done with numbers that are simple algebraic combinations of well-known elementary and special functions, and there are heuristics that work quite well for deciding predicates about such numbers. Calcium will be able to give a definitive answer at least in simple cases (for example, proving \(16 \operatorname{atan}(\tfrac{1}{5}) - 4 \operatorname{atan}(\tfrac{1}{239}) = \pi\) or \(\sqrt{5+2\sqrt{6}} = \sqrt{2}+\sqrt{3}\)), and will simply answer “Unknown” when its heuristics are not powerful enough.

How does Calcium compare to ordinary numerical computing?

Calcium is far too slow to replace floating-point numbers for 99.93% of scientific computing. The target is symbolic and algebraic computation. Nevertheless, Calcium may well be useful as a tool to test and enhance the capabilities of numerical programs.

How does Calcium compare to Arb arithmetic?

The main advantage of Calcium over ball arithmetic alone is the ability to do exact comparisons. The automatic precision management in Calcium can also be convenient.

Calcium will usually be slower than Arb arithmetic. If a computation is mostly numerical, it is probably better to try using Arb first, and fall back on an exact calculation with Calcium only if that fails because an exact comparison is needed.

How does Calcium compare to symbolic computation systems (Mathematica, SymPy, etc.)?

Calculating with constant values is only a small part of what such systems have to do, but it is one of the most complex parts. Existing computer algebra systems sometimes manage this very well, and sometimes fail horribly. The most common problems are 1) getting numerical error bounds or branch cuts wrong, and 2) slowing down too much when the expressions get large. Calcium is intended to address both problems (through rigorous numerical evaluation and use of fast polynomial arithmetic).

Ultimately, Calcium will no doubt handle some problems better and others worse, and it should be considered a complement to existing computer algebra systems rather than a replacement. A symbolic expression simplifier may use Calcium evaluation as one of its tools, but this probably needs to be done selectively and in combination with many other heuristics.

Why is Calcium written in C?

The main advantage of developing Calcium as a C library is that it will not be tied to a particular programming language ecosystem: C is uniquely easy to interface from almost any other language. The second most important reason is familiarity: Calcium follows the design of FLINT and Arb (coding style, naming, module layout, memory management, test code, etc.) which has proved to work quite well for libraries of this type.

There is also the performance argument. Some core functions will benefit from optimizations that are natural in C such as in-place operations and fine-grained manual memory management. However, the performance aspect should not be overemphasized: Calcium will spend most of its time in FLINT and Arb kernel functions and this would probably still be true even if it were written in a slower language.

There are certainly types of mathematical functionality that will be too inconvenient to implement in C. Our intention is indeed to leave such functionality to projects written in Python, Julia, etc. which may then opt to depend on Calcium for basic operations.

What is the development status of Calcium?

Calcium is presently in early development and should be considered experimental software. The interfaces are subject to change and many important functions and optimizations have not been implemented. A more stable and functional release can be expected in late 2021.