You are in a pub. The former maths teacher opposite you is playing with a hexaflexagon; the PhD student next to you is showing the statistician next to her how to pair up the wings on a 4×4×4 Rubik’s cube; and you have just worked out that a square based pyramid made with equilateral triangles (and a square) has twice the volume of a tetrahedron made with the same triangles.

Maybe this sounds too good to be true, but this happens. Monthly. And not just in London: Guildford, Sheffield, Cheltenham, Norwich, Dublin, New York, Bombay, Edinburgh and 28 more towns and cities play host to monthly pub maths.

The initiated will know that I am talking about MathsJam, started in 2008 by stand up mathematician Matt Parker as a place for mathematicians to meet and share puzzles. As everyone brings puzzles along from a wide range of sources, there is always something new and interesting. For example, this puzzle was passed around at a recent MathsJam:

### #38: Eight Peas

There are eight cups, with one pea in each cup. You are allowed to move a pea by picking up the pea in a pot with only one pea and jumping it to the left or the right over two peas into a pot with only one pea in it. For example, the following moves are allowed:

Starting with your eight cups, can you make four moves?

The idea is that you turn up with some puzzles and swap them around: other people work on yours and you work on theirs. But you don’t just get puzzles in return: mathematical games are also popular at MathsJam. One game which has been spotted at many MathsJams is Noughts and Crosses, or Tic-Tac-Toe.

Obviously, I’m not talking about boring old always-a-draw Noughts and Crosses. I’m talking about Double Noughts and Crosses. It starts on a grid like this:

It’s just like a normal Noughts and Crosses board, except a smaller grid has been drawn in each box. Each of the smaller grids works like a normal Noughts and Crosses game. The game can be won by winning three of these smaller grids in

a row.

That’s not all though: there is one more rule which makes the game very interesting. Each of the smaller grids is used like a map of the large grid and where I play limits where you can play. This is easiest to explain with an example opening.

I am crosses; you are noughts. I’m going first. This first move can be anywhere. I play here:

I played in the bottom left corner of a small grid. This means you must play in the bottom left grid with your next move:

You decide to go here:

Similarly, as you have chosen the middle right of a small grid I must now play in the middle right grid:

And so the game continues.

Later in the game, if you try to make me play in a grid which one of us has already won, then I can play anywhere. It’s at this point where the game really comes to life, with each of us trying to force the other to let us play anywhere.

MathsJams are held on the second last Tuesday of every month. The next one is on 19^{th} May, when I will be at the London MathsJam ready to challenge you to a game of Double Noughts and Crosses. Or if you’re not going to be in London in April, challenge someone at your nearest Jam to a game (check the map of MathsJam locations). And don’t forget to share the puzzles and games you discover with us in return!

*Don’t forget to print off our May puzzle page to take to MathsJam.*

* Future London MathsJam events will be listed on our events page. You can send us your puzzles and games at puzzles@chalkdustmagazine.com or @chalkdustmag and you might see one appear in a future edition or on our website.*