# AbstractAlgebra: A Julia package for algebra

This is the first in a series of blog posts we will be writing on aspects of the new OSCAR Computer Algebra System we are writing.

In this blog we’ll be covering a new package we have created as part of the OSCAR project, called AbstractAlgebra.jl.

The AbstractAlgebra package is written entirely in Julia and serves two purposes. The first is to implement generic algorithms in abstract algebra and the second is to outline a set of interfaces that must be implemented in order to make use of these generics.

## Example notebook

Before we begin, I’ve produced an example Jupyter notebook, showing off a small selection of the AbstractAlgebra features.

You can find it on our Examples page

You can view it immediately in rendered form by clicking on the AbstractAlgebra notebook.

Now we can discuss some of the rationale and features of AbstractAlgebra.

## What are generic algorithms?

If one requires polynomials over the integers, for example, one can make use of a specialised fast C/C++ or even a specialised Julia implementation for the job. But what happens if you then want to work in the fraction field of that polynomial ring, or with power series over that fraction field?

Clearly there is a combinatorial explosion in the number of groups, rings, fields, modules and other structures that algebraists want to consider. It is exceptionally unlikely that a dedicated C/C++ library exists for all the rather complicated algebraic objects one wants to work with.

Computer algebra systems solve this problem by providing generics. For example, AbstractAlgebra provides fraction fields and power series over any ring, so long as a certain Ring Interface is implemented.

## What interfaces exist?

The following is a list of all of the interfaces AbstractAlgebra currently spells out:

## What generic constructions does AbstractAlgebra provide

AbstractAlgebra starts with some basic objects, such as Julia BigInts or rationals or BigFloats, or with any type provided by a Julia package that satisfies a simple interface and then implements the following generics over those types:

• Fraction fields
• Dense univariate polynomials
• Sparse distributed multivariate polynomials
• Power series (absolute and relative)
• Laurent series
• Puiseux series
• Residue rings and fields
• Matrices
• Maps, cached maps and maps with inverse

Abstract Algebra also provides the following group functionality (limited to the full symmetric group), with permutations represented internally using any specified Julia integer type:

• Permutation groups
• Characters
• Young tableaux

Of course, everything is recursive, so that you can take a fraction field over a polynomial ring over a residue ring over a polynomial ring over the integers, for example.

## What algorithms does AbstractAlgebra implement?

AbstractAlgebra has been under development for a few years and already implements a large number of algorithms. Some of the highlights are as follows:

• Univariate polynomials : Ducos’ algorithm for resultant, generic interpolation over an integral domain, the pseudo-remainder GCD algorithm for polynomials over a Euclidean domain.

• Laurent and Puiseux series : implementation stores a valuation, scaling factor and precision, so that a series of the form $x^{-1} + 3x^2 + 7x^5 + O(x^6)$ would be stored with a valuation of $-1$, a scaling factor of $3$ and a precision of $6$. This means that only $3$ coefficients are stored, instead of the usual $7$.

• Sparse distributed, multivariate polynomials : heap based algorithms of Johnson, Monagan and Pearce for fast generic multiplication, division and powering of multivariate polynomials. In some cases, our generic Julia implementation is competitive with certain specialised C implementations for basic arithmetic.

• Dense matrices : reduced row echelon forms, linear system solving, minimal and characteristic polynomial, Smith normal form, Hermite normal form over a Euclidean domain, LU decomposition, nullspace, numerous fast determinant algorithms, Popov form and much more.

## What if we want fast C implementations as well?

AbstractAlgebra provides generic implementations which work as generally as possible. Apart from the distinction of working over a ring, field or Euclidean domain, more specialised algorithms are not implemented.

However, we have also been writing a number of packages which provide very specialised C implementations for specific rings and fields.

The most straightforward of these is Nemo.jl. It makes use of the Flint, Arb and Antic C libraries to provide highly optimised C implementations of finite fields, number fields, real and complex numbers, padics, and all the usual rings and fields, such as the integers, rationals, integers mod n, and so on.

Nemo.jl implements the interfaces set out in AbstractAlgebra.jl and therefore any AbstractAlgebra generic algorithm can be used over any Nemo.jl domain.

Moreover, when Nemo.jl is loaded instead of AbstractAlgebra.jl, polynomials over the integers use the highly optimised routines from Flint.

Over the course of the next few weeks and months, we’ll be introducing other libraries we are working on which implement AbstractAlgebra interfaces.

Of course AbstractAlgebra.jl is fully usable on its own, without Nemo.jl and C dependencies. It is great when one wants pure Julia code, or where the C library dependencies of Nemo.jl are not wanted, e.g. when interfacing some other package to AbstractAlgebra.jl for its generics capability.

This “pure Julia” feature of AbstractAlgebra.jl is something that has been requested by developers in the Julia community, and we hope it will attract volunteer contributors to further develop its capabilities in the area of Abstract Algebra.

## What is planned for the future of AbstractAlgebra?

AbstractAlgebra is already quite stable, and we are using it extensively in other components of OSCAR that we are working on.

However, there are many other things we’d like to add in the future.

We’ve made a list of things we definitely want. See

Maybe this list inspires others who would like to contribute something to the AbstractAlgebra.jl project!

One of the biggest features not yet supported by AbstractAlgebra, but planned for the future, is non-commutative algebra. Currently, AbstractAlgebra assumes all rings are commutative (matrices and groups are of course not).

## Documentation

Recently, we’ve spent a lot of time documenting all the AbstractAlgebra interfaces and all of the generic functionality it provides.

The complete documentation for AbstractAlgebra.jl is available here:

AbstractAlgebra.jl documentation

Of course, there are sure to be errors in the documentation. Pull requests and bug reports are welcome on the GitHub issue tracker!

## AbstractAlgebra’s role in OSCAR

The OSCAR project is a very large project with many working parts. Part of the integration work being done in OSCAR makes use of Julia, and indeed some of the packages we are implementing are written entirely in Julia (e.g. Nemo.jl and Hecke.jl).

AbstractAlgebra.jl is a package that most, if not all, Julia components of OSCAR will use. It spells out the common interfaces that are required for generics to work. Then it provides the generic algorithms back to those other systems. This is possible because of the ability to embed Julia in native programs.

## Scratch an itch

Do you have an itch you want to scratch and AbstractAlgebra might get you part of the way there? We’d love to hear from you.

AbstractAlgebra is considered very stable at this point, and we feel that now is the right time to be announcing it and encouraging pull requests. We believe that the code that is already there gives a good template for future additions; it should be quite clear how to follow the pattern.

Of course, it is better if we know what’s coming, rather than to have large code dumps that require significant work to harmonise them with other code that is there or on its way. So we ask contributors to talk to us early and often, so we can be as helpful as possible.

Look back here often for updates on the OSCAR project, as we release them. You can already find a lot of interesting news on the new website that this blog is a part of!