The cover picture shows a 22° halo seen in Preston on Sunday 28 October 2001 at 10 am. In contrast with a rainbow, the red band is on the inside rather than the outside, the colour bands are much thinner, and the arc is centred round the sun instead of being opposite to the sun. Also, it is not raining. Haloes occur on still sunny days when there is plenty of cirrus cloud. It is possible to see a full circle depending on how high the sun is in the sky. This never happens with a rainbow except when seen from an aeroplane. Sometimes the colours are particularly vivid at the 3 o’clock and 9 o’clock points. These vivid areas are called parhelia or mock suns. Sometimes the parhelia are all that can be seen.

In this paper we shall consider two situations in triangle geometry when equilateral triangles appear and then show that they are closely related.

In the first (known as the Napoleon theorem) equilateral triangles BCAT, CABT, and ABCT, are built on the sides of an arbitrary triangle ABC and their centroids are (almost always) vertices of an equilateral triangle ANBNCN (known as a Napoleon triangle of ABC; see Figure 1).

In a recent paper on Horner’s Method [1], which includes a compact method for dividing expressions, we mentioned that some Computer Algebra Systems (CASs) such as DERIVE could be used to make the calculations, but that such programs, even when obtained through educational establishments, are overly expensive especially when most PCs have spreadsheets on them that could equally well do the calculations. Here we describe the use of an Excel spreadsheet to divide one expression by another, first by the method of detached coefficients and second by Horner’s Method of Synthetic Division (or simply synthetic division). A third example uses Horner’s Method to replace x by (x + c) to form a new expression [2], useful in the determination of the roots of a polynomial.

The familiar fact that we may cut any triangle into three polygonal pieces which can be rearranged to form a rectangle is one of the most elementary problems of polygonal dissection. This fact is shown in two ways in Figure 1 where both sides of the rectangle are predetermined after setting up the triangle: one side of the triangle and half of the altitude to this side, or one altitude and half of the side at right angles to it.

Home to just over five million souls, Scotland is the most sparsely populated part of Britain. The people are overwhelmingly white (some 98.7%) and English speaking. Levels of deprivation vary considerably across the country as a whole. Some 20% of the school population was entitled to free school meals in 1995, though the figure was twice as high in the City of Glasgow, where life expectancy is 10 years below that of affluent parts of the south of England. In July 1997 proposals were presented for the creation of a Scottish parliament. Whilst the Westminster parliament would ‘remain sovereign’, many powers would be devolved to Edinburgh, including those relating to virtually every aspect of education. So today, the Scottish Executive Education Department (or SEED) administers Scottish Executive policy for pre-school and school education in co-operation with local authorities that are responsible for providing school education in their areas. No less than 96% of youngsters are educated in state schools. Schools associated with religious groups including the Roman Catholic Church were incorporated into the state system in the 1920s. The annual cost of running the whole education system is a little under £5 billion or some 9% of Scottish GDP [1, p. 17].

Given a triangle with vertices P1, P2, P3 its Fermat point is defined as that point F which minimises the sum P1F + P2F + P3F. It is known that if each of the angles P1, P2, P3 is less than 120°, then F will be the point inside the triangle at which each pair of P1, P2, P3 subtend an angle of 120°, while if any of the angles at P1, P2, P3 exceed 120° then F will coincide with that vertex – see Figure (1).

This article is a version, adapted to the written word, of an invited lecture given to the annual conference of Heads of Departments of Mathematical Sciences in universities, at the University of Birmingham, on 20 April 2001. It is offered here in the belief that an indication of the thinking behind the provision of Masterclasses for schoolchildren may also be of interest to schoolteachers themselves. I have been grateful for the very strong support which teachers have given, over ten years, to the Masterclasses which I have organised.

One of the long-standing problems of number theory, appealing to professional and recreational mathematicians alike, is the existence of Mersenne primes. These puzzling primes, for example 7, 31, 127 and 8191, are of the form 2P - 1, where p is itself a prime. The problem of their existence originated some 2400 years ago with the early Greek mathematicians' quest for the so-called perfect numbers, those like 6 and 28 which are the sum of their proper divisors. The connection was given in Euclid's Elements in 300 BC: if 2P - 1 is prime, then 2P-1(2P - 1) is a perfect number. Much later, Euler proved that all the even perfect numbers correspond to Mersenne primes. So the interest for many years has been in finding Mersenne primes. Only 39 are known, including several monsters discovered in recent years using thousands of PCs coordinated via the internet (see [1] for information on the project). Many of us would like to know if there is any way of predicting which exponents yield Mersenne primes and whether there is an infinite number of them.

The roots of a polynomial can be represented as points in the complex plane. The time value of money (TVM) equation that is commonly used in finance is a polynomial equation. (See the appendix for a short description of the TVM equation and an example of its use in finance.) In [1] and [2] it is shown that concepts from financial mathematics can be obtained from the pattern of the roots of the TVM equation. The concepts are given in terms of distances between the roots and other salient points in the plane. This note shows that this particular polynomial, and the technique, can be applied more generally. When a series of data is fed into the coefficients of the polynomial, the mean and standard deviation of the data are seen in the complex plane as combinations of distances between the roots and other salient points. The results are aesthetically pleasing as well as mathematically interesting.

We have developed a novel diagrammatic approach for understanding and teaching probability theory — Probability Space diagrams [1]. Our studies of learning and instruction with Probability Space (PS) diagrams have demonstrated that they can significantly enhance students' conceptual understanding. This article illustrates the utility of PS diagrams by applying them to the explanation of some difficult concepts and notoriously counterintuitive problems in probability. We first outline the nature of the system.