# Constructing the cover of Issue 10

Ever since Chalkdust Issue 10 was released in October we’ve been admiring the beautiful cover, which features artwork created by Samira Mian. Samira is inspired by patterns from Islamic geometry, which use simple compass and ruler constructions to create intricate tiles that can be repeated in a variety of ways. This article from Issue 10 discusses the history of patterns in Islamic geometry, and their connection to modern mathematics. If you’re curious about how these patterns are formed, and want to try constructing one yourself, Samira has shared with us a step-by-step guide to constructing the tile that forms the basic unit of the cover image, inspired by the Persian architect and geometer Ali Reza Sarvdalir. Continue reading

Our latest edition, Issue 10, is available now. Enjoy the articles online or scroll down to view the magazine as a PDF.

## Features

• ### Can computers prove theorems?

And will we soon all be out of a job? Kevin Buzzard worries us all.
• ### In conversation with Clifford Cocks

We chat to the crypto chief about inventing RSA... but not being able to tell anyone
• ### On √2

Yiannis Petridis connects square roots and continued fractions
• ### Spotlight on: Pamela Harris

Pamela E Harris's story, as told by Talithia Williams
• ### Artificial music

Carmen Cabrera Arnau explores the use of AI in composition
• ### They might not be giants

Angela Brett might not be standing on their shoulders
• ### On the cover: Islamic geometry

Explaining the mathematics of tiling, and the cover of Issue 10

• ### Secrets, surveys and statistics

Paula Rowińska uses mathematics to answer some awkward questions
• ### Curiosities of linearly ordered sets

Andrei Chekmasov explores order and infinity

## Fun

• ### Prize crossnumber, Issue 10

Can you solve it?
• ### Dear Dirichlet, Issue 10

Letter writing, hospital visits, and getting the family active are among the topics of discussion in this issue's Dear Dirichlet advice column
• ### Comic: The Inverse Homotopy, part 7

Part 7 of our mathematical comic's adventure
• ### Which mathematician are you?

How do you like your numbers? Prime? Positive but infinitely small? Find out which famous mathematician you are
• ### What’s hot and what’s not, Issue 10

Fashion is fleeting, Chalkdust regulars are not.
• ### Top Ten: issues of Chalkdust

The definitive chart of the best issues of Chalkdust
• ### Page 3 model: Bees

You won't bee-lieve it
• ### Top ten vote issue 10

Vote for your favourite picture of a scorpion
• ### How to make: tessellating shortbread

Tasty and mathematical

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# Can computers prove theorems?

How do we prove that $2 + 2 = 4$? At school, this might have been taught to you in the following way. You were given a box of little plastic cubes, two cubes were put in one of your hands and then two more cubes in the other, and you were challenged to count how many cubes you had in total.

But this doesn’t really prove that $2 + 2 = 4$: it proves that 2 plastic cubes + 2 plastic cubes = 4 plastic cubes. You could try it again with pencils and show that 2 pencils + 2 pencils = 4 pencils, and after you’ve tried it with sufficiently many things you might become convinced that there is an underlying pattern. But is this a proof that $2 + 2 = 4$? Proving it like this feels a bit like an experimental science—it works with cubes, it works with pencils, and this is evidence that it works in general. I think we are all pretty confident that, whatever the actual rules of maths are, they probably don’t mention pencils.

But what are the rules of maths? Are there any rules at all, or do we all just have some inbuilt intuition as to what constitutes a valid mathematical argument? Before the 1900s, people worked intuitively, and there was broad agreement as to what constituted a correct argument. But as people began to do more complex and abstract mathematics, this approach became problematic, because people’s intuitions could differ. Ask a room full of teenagers whether $0.9999999\ldots=1$, and you will get different opinions. This is because different people have different intuitions about what the real numbers actually are. Differences of opinion as to whether arguments were valid forced mathematicians into actually writing down an official rulebook: the axioms of maths. Continue reading

# In conversation with Clifford Cocks

Throughout history, people have wanted to communicate in secret. But for a long time, the need for sender and recipient to agree on a way to encode their message (a ‘key’) meant that secure communication was costly, and mostly used by the military. But in the 1970s new mathematical ideas paved the way for public-key cryptography, a communication strategy that doesn’t rely on a mutually agreed key. If you’ve ever banked or shopped online then you’ve used public-key cryptography, most probably a type called the Diffie–Hellman protocol. (If you want to brush up on Diffie–Hellman, this is a great time to dig out Axel Kerbec’s article Hiding in plain sight from Chalkdust issue 09.) One of the lesser-known figures in the story of public-key cryptography is Clifford Cocks, a former chief mathematician at Britain’s GCHQ (the Government Communication Headquarters). Cliff’s relative anonymity is because, due to the secretive nature of his employer, his contribution was not made public for 24 years. We caught up with him via video call to find out what it felt like to have cracked the code, but kept it secret. Continue reading

# On √2

One of the first theorems lots of students see is that the square root of two is irrational (ie not a fraction). Therefore, we cannot restrict our attention to rational numbers only. Clearly $\sqrt{2}$ is a number we must have, as by Pythagoras’ theorem it represents the length of the hypotenuse of a right-angled isosceles triangle with vertical sides $1$. What the theorem says is that $\sqrt{2}$ is never $x/y$ with $x$, $y$ integers. Or, to put it another way, $$2\ne \frac{x^2}{y^2}\Leftrightarrow x^2\ne 2y^2,$$ that is, the square of an integer is never twice the square of another integer. However, they are both integers. The closest they can be apart is $1$, ie $$x^2-2y^2=\pm 1.$$ This is the simplest form of Pell’s equation: $x^2-Ny^2=\pm 1$. When $N =2$, one can easily find a solution in the integers $(x, y)=(1, 1)$. With a little more thinking we find the solutions

 $(x, y)$ Why? $(1, 1)$ $1^2-2\cdot 1^2=-1$ $(3, 2)$ $3^2-2\cdot 2^2=+1$ $(7, 5)$ $7^2-2\cdot 5^2=-1$ $(17, 12)$ $17^2-2\cdot 12^2=+1$

# Artificial music

Out of all the words in the English dictionary, art is possibly the one with the most debatable definition. In his 1897 book What Is Art?, Russian writer Leo Tolstoy argued that “art begins when a person, with the purpose of communicating to other people a feeling they once experienced, calls it up again within themself and expresses it by certain external signs”. An important aspect in Tolstoy’s argument is that of the artist’s sincerity—that is, the extent to which the artist has experienced the feeling that they are expressing—which is crucial in determining the appreciation of the work by others.

Contrary to Tolstoy’s belief is the one popularised by the French writer Théophile Gautier in the early 19th century, summarised in the slogan l’art pour l’artart for art’s sake. For Gautier, the intrinsic value of a work of art has to be completely detached from any sort of sentimental, social or moral context.

New technologies add a layer of complexity to the old and neverending discussion about what should be considered art. What would the conversation between Tolstoy and Gautier be like after having been presented with one of Emmy’s musical compositions? Emmy, short for ‘Experiments in Music Intelligence’, was created in 1981 by David Cope, nowadays professor emeritus at the University of California, Santa Cruz. Cope, who was suffering from composer’s block, wanted to build software able to generate new material in line with his own pieces, using these pieces as the main input for the software. However, due to the lack of personal works, he started by taking the pieces of various classical composers as the input for his computer programs instead. After spending some time perfecting Emmy, Cope was able to produce, in a matter of minutes, thousands of new instances of music in JS Bach’s style. This resulted in the 1993 release of Bach by Design, one of his several computer-generated music albums.

Since Cope’s days, music-generating systems using artificial intelligence have experienced big advances. Nowadays, there are all sorts of user-friendly systems: IBM Watson Beat, Google Magenta’s NSynth Super, Jukedeck, Melodrive, Spotify’s Creator Technology Research Lab, Amper Music, and so on. Some music systems, like Amper, have explicitly been taught the rules of music theory. However, most AI music systems use artificial neural networks to generate output. The neural networks identify patterns from the multiple samples of source material they are fed with. These patterns are then used to create new music in the form of an audio file or a music score. While some systems will simply create a melody from a given note, others are able to harmonise a given melody.

A chorale harmonisation or a chorale is a musical piece traditionally intended to be sung by a congregation during a German Protestant service. It is often written for soprano, alto, tenor and bass. The soprano is the voice that holds the melody, which is usually a Lutheran hymn tune, while the other three voices provide the harmony.

For a taste of what AI is capable of doing, you can have a look at the Google Doodle from 21 March 2019, celebrating Bach’s 334th birthday. Coconet is the machine learning model that makes this Doodle work. Trained with a relatively small dataset of 306 choral harmonisations by Bach, Coconet can harmonise a melody entered by the user in Bach’s contrapuntal style in a matter of seconds. The mechanisms behind the Doodle are explored in the following section.

## Coconet in a nutshell

Coconet’s task involves taking incomplete musical scores and filling them up with the missing material. For the result to be loyal to Bach’s style, Coconet needs to first be trained to know what is the ‘right’ style. This training is done by randomly erasing some notes from the original chorales composed by Bach and asking Coconet to reconstruct the erased notes. A rank is given to quantify the accuracy of Coconet’s version with respect to Bach’s. Coconet will then be encouraged to repeat high-ranked guesses in future reconstructions of incomplete music scores, while trying to avoid low-ranked guesses.

So how is the music extracted from probability distributions? One could think naively that it is OK to just pick the pitch which corresponds to the highest probability assigned to the missing notes for each voice independently. However, Bach chorales are all about harmony and harmony is all about interactions between notes; the melodic lines of the different voices cannot be considered in isolation.

To account for these interaction effects, there are several solutions. Perhaps the most obvious one would be to assign the highest probability pitches to one of the voices, and then feed Coconet with this new version of the incomplete chorale. The model would update the probability distributions for the other voices. The process could then be iterated until all the voices are complete. Although it is simple, this solution is not ideal; very different results might be obtained depending on which voice is completed first.

Coconet opts for a more robust solution. At first, all the pitches in the incomplete chorale are filled up simultaneously according to the highest probabilities for each of the individual voices. But this result is just taken as a draft. Then, some of the guesses are randomly erased and the new incomplete chorale is fed into Coconet again. New probability distributions are obtained for the new gaps. The process, called blocked Gibbs sampling, is repeated until the probability distributions given at consecutive iterations of the process are similar enough to always give the same pitch.

The diverse opinions about the final products are as interesting, if not more, as the mechanisms behind AI-generated music. The audience’s reaction to artificially generated music was spectacularly tested at the University of Oregon in 1997. There, the pianist Winifred Kerner performed three pieces: one written by her husband, the composer Steve Larson; another one written by Bach; and the last one, generated by Emmy. After her performance, the audience was asked to guess which was which. To Larson’s despair, the audience concluded that his composition had been created by Emmy and that Emmy’s work was genuine Bach.

Larson was not the only one feeling uncomfortable about the fact that Emmy had been able to fool a whole audience. American professor of cognitive science Douglas Hofstadter, author of the 1979 Pulitzer prize-winning book Gödel, Escher, Bach, had argued a machine “would have to wander around the world on its own, fighting its way through the maze of life and feeling every moment of it” in order to produce anything similar to the masterpieces. In a 1997 article published by the New York Times, he claimed that the only comfort he could take from Larson’s experiment in front of the audience was that “Emmy doesn’t generate style on its own. It depends on mimicking prior composers”.

# Black mathematician month 2019

This is our third year of running Black Mathematician Month and we could not be more pleased for this time to come round again.

We believe that diversity is important in all fields. I work on mathematical modelling applied to biological and medical problems, and one of the most exciting advances on the horizon is the idea of personalised medicine. This is the idea that medicine should be tailored to the individual. When medicine is prescribed, the questions of gender, age and ethnic background, among others, should be taken into account in order to establish the most successful treatment. However, for this goal to be possible we need to consider a wide range of different people, and the most direct way to do this is to have a diverse group of people involved in the research. If mathematics is supposed to benefit all of society, then surely the same is true for us.

## Why focus on black mathematicians?

Usually, when we speak about diversity in mathematics we focus solely on gender. However, there are female mathematics role models in the public eye—Rachael Riley on Countdown, Hannah Fry on Radio 4, are but a few of the more famous female mathematicians. But how many black mathematicians can you name? Stepping onto any university campus, it’s easy to notice that the research community is not ethnically representative of the population. Similar issues have been raised in the ‘Decolonising the curriculum’ movement and were noted in reports by the charity Advance HE. Here, although it was noted that 23.6% of mathematics students are BME (Black and minority ethnic), the real issue is hidden by lumping all BME students together. A recent report showed that, shockingly, only 0.6% of all UK professors are black, compared to about 4.6% of the population in England and Wales.

## Towards diversity in higher education

People are trying to tackle the problem. It was announced that Stormzy will be funding another round of scholarships at Cambridge this year. Leading Routes run a range of different events and have recently published the `Broken pipeline’ report into why there are not more black students with fully funded PhD places.  Throughout February 2019, Mathematically gifted and black published the profiles of 28 black mathematicians. Chalkdust also wants to do our part. The aim of Black mathematician month is to raise the profile of black mathematicians, in order to provide role models for the next generation who aspire to study mathematics and to generate a conversation about the role of diversity in mathematics.

In previous years we have published interviews and articles written by black mathematicians, which you can read here. This year, throughout October we will be giving our Twitter account up for ‘take-overs’ by black academics who are leading the conversation on what we can do to improve diversity in the mathematical sciences. Further, from the past few years we have realised that promoting diversity once a year is not enough. This is why we are planning two events for early 2020. One will be for year 9 and 10 school children in London, with a series of workshops and talks to encourage them to consider carrying on studying mathematics. The second will be a networking event for black students, academics and those in industry working in the mathematical sciences. Keep an eye out for these events!

Black mathematician month is something we feel strongly about, however it’s also something that we need help in running. If you would like to get involved or would like to tell us about any events you are running for Black mathematician month, please get in touch!

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# Crossnumber winners, issue 09

With issue 10 of Chalkdust fast approaching, it’s time to announce the winners of the Chalkdust prize crossnumber #9! Before we reveal the winners, here is the solution of the crossnumber.

 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 9 6 4 8 8 9 3 9 4 6 6 1 6 9 4 1 3 1 1 1 7 2 5 9 5 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 9 2 2 8 2 0 8 7 9 1 9 2 2 2 9 2 3 5 8 3 8 3 6 1 5 3 3 7 1 9 9 5 7 1 2 2 9 1 9 2 2 2 9 2 8 1 6 3 4 2 1 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 8 9 9 4 7 4 3 2 7 4 2 3 4 0 7 4 1 2 3 6 6 4 8 3 3 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9

The sum of the across clues was 2222222406182591.

There were 100 entries, 66 of which were correct. The randomly selected winners are:

1. Matt Hutton, who wins a £100 Maths Gear goody bag,
2. George Lambert, who wins a Chalkdust T-shirt,
3. Sami Wannell, who wins a Chalkdust T-shirt,
4. Deborah Tayler, who wins a Chalkdust T-shirt.

Well done to Matt, George, Sami and Deborah, and thanks to everyone else who attempted the crossnumber. See you shortly in issue 10…