Throwaway Digits

Tomorrow, my PhD student He Li will present our paper Digit Elision for Arbitrary-accuracy Iterative Computation (joint work with James Davis and John Wickerson) at the IEEE Symposium on Computer Arithmetic in Amherst, MA.

Readers of this blog may remember that we previously came up with a neat way of computing arbitrarily precise values of arbitrarily deep iterations of an iterative real-number computation, while only using constant-area compute hardware. This latest paper extends our previous work in the following way.

In our previous work, we computed every digit of every iteration of the computation. While for any computable real function this will give a correct result, it tends to be wasteful in practice. There are two reasons it’s wasteful. Firstly, often the reason we’re computing an iteration is because that iteration converges. Convergence can be seen as agreement in most-significant digits – after a while they don’t change. So why do we recompute them? We see this again and again in standard numerical computing – each iteration might add just a couple of new correct digits, but we still end up wasting time and energy computing all of the digits in each iteration, even the stable ones. Secondly, not all iterations may contribute equally to the overall error resulting from early termination. This paper addresses these two issues.

The first, and more general, issue is the wastefulness of computing stabilised digits. But just because they look stable, are they really stable? Maybe we’ve stabilised to 0.9, 0.99, 0.999, 0.999, and then one more iteration might kick us over to 1.0001. So can we really afford not to recompute most-significant digits? Ercegovac‘s Online Arithmetic comes to our rescue again! If we compute in an appropriate redundant number representation, then we can prove that stability of digits means we don’t need to consider them any more. This is our first contribution – to recognise this and utilise it within an appropriately modified computational architecture.

The second, and more specific, issue is that some digits are effectively ‘don’t care’. In this paper, we only analyse the specific case of stationary iterative methods (Jacobi, SOR, etc.) for this kind of digit. We show that, in these cases, for a fixed digit budget (e.g. “compute at most D digits across all iterations”), you should allocate these digits by computing a constant more digits each iteration. This constant can be estimated from the infinity norm of a certain matrix involved in the computation. Again, we modify our hardware architecture to take advantage of this pattern.

The end result is that we end up tracing out a corridor of digits, shown in the figure below, where the vertical axis is iteration and the horizontal axis is precision / digit number. Some digits have provably stabilised and no longer need computation (marked “), some are irrelevant don’t cares (marked X). This corridor radically improves the storage requirements of the original ARCHITECT scheme.

Screen Shot 2018-06-25 at 14.07.29

Hardware for Rational Functions

Next Tuesday, my collaborator Silviu-Ioan Filip will present some of our recent work with Nicolas Brisebarre, Miloš Ercegovac, Matei Istoan and Jean-Michel Muller at the IEEE International Symposium on Computer Arithmetic.

In the 1970s, Miloš invented a rather nice method called the E-method for evaluating rational functions, i.e. ratios of two polynomials. The basic idea of his method is as follows. We may solve a system of linear equations $Ay = b$ where $A$ is a matrix of a special structure formed from constants $q_i$ together with variable $x$ :

$A = \begin{bmatrix} 1 & -x & 0 & 0 & \cdots & 0 & 0 \\ q_1 & 1 & -x & 0 & \cdots & 0 & 0 \\ q_2 & 0 & 1 & -x & \cdots & 0 & 0 \\ \vdots & \vdots & \ddots & \ddots & \ddots & \vdots & \vdots \\ q_{n-1} & 0 & 0 & 0 & \cdots & 1 & -x \\ q_n & 0 & 0 & 0 & \cdots & 0 & 1 \end{bmatrix}$

If we further choose the vector $b = \begin{bmatrix} p_0 & \cdots & p_n \end{bmatrix}^T$ , then it turns out that the first element of the solution vector is the rational function $\frac{p_n x^n + \cdots + p_0}{q_n x^n + \cdots + q_0}$ .

So we can use this to evaluate such rational functions. On the face of it, that doesn’t seem very interesting: why would we go to the bother of solving a system of linear equations to evaluate a rational function?

The answer lies in the combination of this idea with another one of Miloš’s key contributions, the idea of online arithmetic – computing results most-significant-digit first. In fact, if the matrix $A$ is sufficiently well conditioned then we may use a stationary iterative method to solve the system of equations in such a way that it produces one new correct digit of the solution for each iteration of the method, leading to very efficient evaluation.

Our paper at ARITH makes two novel contributions. Firstly, we show how to find such a matrix $A$ that is sufficiently well conditioned and for which the solution is close to a given function we’re trying to approximate, improving on the previous technique of Brisebarre et al. Secondly, we show how this method can be efficiently implemented in modern FPGA hardware, when aiming for high throughput.

The main domain of interest will be functions where rational approximation provides a much better fit than polynomials, as the computation required essentially provides rational computation for the price of polynomial computation. A buy-one-get-one-free offer, if you will.

I’m pleased to say that both the rational approximation generator and the hardware IP core generator will soon be open-sourced. Watch this space! Edit: I’m pleased to say this is now available at https://github.com/sfilip/emethod.

Structures in Arithmetic Teaching Tools

Readers of this blog will know that beyond my “day job”, I am interested in early mathematics education. Partly due to my outreach work with primary schools, I became aware of several tools that are used by primary (elementary) school teachers to help children grasp the structures present in arithmetic. The first of these, Cuisenaire Rods, have a long history and have recently come back in vogue in education. They consist of coloured plastic or wooden rods that can be physically manipulated by children. The second, usually known in this country as the Singapore Bar Model, is a form of drawing used to illustrate and guide the solution to “word problems”, including basic algebra. Through many discussions with my colleague, Charlotte Neale, I have come to appreciate the role these tools – and many other physical pieces of equipment, known in the education world as manipulatives – can play in helping children get to grips with arithmetic.

Cuisenaire and Bar Models have intrigued me, and I spent a considerable portion of my Easter holiday trying to nail down exactly what arithmetic formulae correspond to the juxtaposition of these concrete and pictorial representations. After many discussions with Charlotte, I’m pleased to say that we will be presenting our findings at the BSRLM Summer Conference on the 9th June in Swansea. Presenting at an education conference is a first for me, so I’m rather excited, and very much looking forward to finding out how the work is received.

In this post, I’ll give a brief overview of the main features of the approach we’ve taken from my (non educationalist!) perspective.

Firstly, to enable a formal study of these structures, we needed to formally define how such rods and diagrams are composed.

Cuisenaire Rods

These rods come in all multiples up to 10 of a single unit length, and are colour coded. To keep things simple, we’ve focused only on horizontal composition of rods (interpreted as addition) to form terms, as shown in an example below.

In early primary school, the main relationships being explored relating to horizontal composition are equality and inequality. For example, the figure below shows that black > red + purple, because of the overhanging top-right edge.

With this in mind, we can interpret any such sentence in Cuisenaire rods as an equivalent sentence in (first order) arithmetic. After having done so, we can easily prove mathematically that all such sentences are true. Expressibility and truth coincide for this Cuisenaire syntax! Note that this is very different to the usual abstract syntax for expressing number facts: although 4 = 2 + 1 is false, we can still write it down. This is one reason – we believe – they are so heavily used in early years education: truths are built through play. We only need to know syntactic rules for composition and we can make many interesting number sentences.

From an abstract algebraic perspective, closure and associativity of composition naturally arise, and so long as children are comfortable with conservation of length under translation, commutativity is also apparent. Additive inverses and identity are not so naturally expressed, resulting in an Abelian semigroup structure, which also carries over to our next tool, the bar model.

Bar Models

Our investigations suggest that bar models – example for $20 = x+2$ pictured below – are rarely precisely defined in the literature, so one of our tasks was to come up with a precise definition of bar model syntax.

We have made the observation that there seem to be a variety of practices here. The most obvious one, for small numbers drawn on squared paper, is to retain the proportionality of Cuisenaire. These ‘proportional bar models’ (our term) inherit the same expressibility / truth relationship as Cuisenaire structures, of course, but now numerals can exceed 10 – at the cost of decimal numeration being a prerequisite for their use. However, proportionality precludes the presence of ‘unknowns’ – variables – which is where bar models are heavily used in the latter stages of primary schools and in some secondary schools.

At the other extreme, we could remove the semantic content of bar length, leaving only abutment and the alignment of the right-hand edges as denoting meaning – a type of bar model we refer to as a `topological bar model’. These are very expressive – they correspond to Presburger arithmetic without induction. It now becomes possible to express false statements (e.g. the trivial one below, stating that 1 = 2).

As a result, we must be mathematically precise about valid rules of inference and axiom schemata for this type of model, for example the rule of inference below. Note that due to the inexpressibility of implication in the bar model, many more rules of inference are required than in a standard first-order treatment of arithmetic.

The topological bar model also opens the door to many different mistakes, arising when children apply geometric insight to a topological structure.

In practice, it seems that teachers in the classroom informally use some kind of mid-way point between these two syntaxes, which we call an `order-preserving’ bar model: the aim is for relative sizes of values to be represented, ensuring that larger bars are interpreted as larger numbers. However, this approach is not compositional. Issues arising from this can be seen when trying to model, for example, $x + y = 3$ . The positive integral solutions are either $x = 2, y = 1$ leading to $x > y$ or $x = 1, y =2$ , leading to $y > x$ .

Other Graphical Tools and Manipulatives

As part of our work, we identify certain missing elements from first-order arithmetic in the tools studied to date. It would be great if further work could be done to consider drawings and manipulatives that could help plug these gaps. They include:

Multiplication in bar models. While we can understand $3x$ , for example, as a shorthand for $x + x + x$ , there is no way to express $x^2$
Disjunction and negation. While placing two bar models side-by-side seems like a natural way of expressing conjunction, there is no natural way of expressing disjunction / negation. Perhaps a variation on Pierce’s notation could be of interest?
We can consider variables in a bar model as implicitly existentially quantified. There is no way of expressing universal quantification.
As noted above, these tools capture an Abelian semigroup structure. We’re aware of some manipulatives, such as Algebra Tiles, which aim to also capture additive inverses, though we’ve not explored these in any depth.
We have only discussed one use of Cuisenaire rods – there are many others – as the recent ATM book by Ollerton, Williams and Gregg makes clear, many of which we feel could also benefit from analysis using our approach.
There are also many more manipulatives than Cuisenaire, as Griffiths, Back and Gifford describe in detail in their book, and it would be of great interest to compare and contrast these from a formal perspective.
At this stage, we have avoided introducing a monus into our algebra of bar models, but this is a natural next step when considering the algebraic structure of so-called comparative bar models.
My colleague Dan Ghica alerted me to the computer game DragonBox Algebra 5+, which we can consider as a sophisticated form of virtual manipulative incorporating rules of inference. It would be very interesting to study similar virtual manipulatives in a classroom setting.

An Exciting Starting Point

Charlotte and I hope that attendees at the BSRLM conference – and readers of this blog – are as excited as we are about our idea of the potential for using the tools of mathematical logic and abstract algebra to understand more about early learning of arithmetic. We hope our work will stimulate some others to work with us to develop and broaden this research further.

Acknowledgement

I would like to acknowledge Dan Ghica for reading this blog post from a semanticist’s perspective before it went up, for reminding me about DragonBox, and for pointing out food for further thought. Any errors remain mine.