# Once Upon a Prime

What an amazing book! I was so excited when I read about ‘Once Upon a Prime’ by Sarah Hart in the Guardian. It coincided with Celeste my 9 year old granddaughter asking ‘Grandma, what is a prime number?’

I replied with the usual explanation: ‘It’s a number that can only be divided by itself and 1’ though I must admit I don’t find that so helpful and prefer ‘prime numbers are numbers that can’t be divided into smaller whole numbers.’

2,3,5,7,11,13, 17,19,23,29,31,41,43,47,53,59,61,67,71,73,79,83,89,97….

All even numbers beyond 2 can’t be prime – by definition they can all be divided by 2!

As numbers increase the fewer prime numbers you will find…but they do go on forever…

Sarah introduced us in a fun way to prime numbers in nursery rhymes and prime number patterns in poetry.

The book is about so much more: ‘The Wondrous Connections between Mathematics and Literature’ and a whole bunch of ideas I could explore with my 9 year old.

Celeste enjoys writing stories and knows about the importance of an arc in the plot but was fascinated by the illustrations of graphs which follow a story’s development.

Giving a whole new view of the story’s progression over time – not just arcs but many more shapes that can be used to both plan the plot of a new story and to analyse the narrative of an existing story. The y-axis goes from bad (-ve) to good(+ve) and the x-axis is time.

In Cinderella for example (p38), the story starts with unhappiness as she is enslaved by the ugly sisters (-ve), then a fairy godmother casts a spell and frees her to go the ball and meet a handsome prince (+ve). At midnight the spell in broken and all is lost as Cinderella returns home sad (-ve) then miraculously the prince finds her and fits her lost slipper (+ve) and they live happily ever after (+ve to infinity).

We also talked about interactive stories and how difficult they are to write. Sarah Hart introduced us to the use of tree structures to represent the number of decision points in the story and illustrating that if you allow 2 decisions at each point in the story, you will never reach the end!

For example in our first attempt at writing a story outline:

Two children are stuck at the top of a volcano and it starts to rumble.

Should they (A) call for a superhero to rescue them or (B) run as fast as they can?

If they call the superhero it leads to further choices C and D

If they run then that leads to E and F

The story keeps diverging C leads to G and H, D leads to I and J, E leads to K and L, F to M and N – it never  reaches an end point.

Sarah explains the use of the theatre tree (p81) as a way of reducing the number of paths and reaching a satisfactory ending – and giving us a much better way of planning that interactive story.

We were also delighted to be introduced to ‘reverse poems’. For example Lost Generation by Jonathan Reed (p88) can be read forwards and backwards and still make sense.

I am part of a lost generation

And I refuse to believe that

I can change the world

This led us to thinking about palindromes in numbers and words, and spent the rest of the day telling each other new ones we’d thought of and laughing. It was such fun!

As Rohan Silva in the Guardian  puts it:

‘Once Upon a Prime is a joyous reminder of the way so much human activity comes from joining the dots between seemingly disparate fields’

Thank you Sarah

Sarah Hart is Professor of Mathematics at Birkbeck College (University of London)