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Aperiodical’s Mathematical Seasonal Gift Guide, 2020

Image by Yvette Fang from Pixabay

Given that it’s conventional to give objects to other people around this time of year, we thought we’d collect together some suggestions for things we think you, a mathematically interested person, might like to buy for your mathematical friends (or add to your list before you send it off to Santa).

Mobile Numbers: Cellular Automaton

In this series of posts, Katie investigates simple mathematical concepts using the Google Sheets spreadsheet app on her phone. If you have a simple maths trick, pattern or concept you’d like to see illustrated in this series, please get in touch.

Spreadsheets are wonderfully versatile, and the fact that I can now carry spreadsheets around on my phone, in my pocket, is a marvellous result of the smartphone boom. I’m always pleased to see so many people wandering around using spreadsheets on their phones (I assume).

So when my Aperiodicolleague CL-P sent me a link to his latest spreadsheet creation, I was pretty excited to find yet another application of spreadsheets – for modelling cellular automata! As we’ve previously written about on this site, cellular automata are systems that have a set of rules to determine how they change over time, on a cell-by-cell basis (spreadsheets are really the natural choice here). While many automata, such as Conway’s famous Game of Life, are 2-dimensional, that’s slightly difficult to represent on a spreadsheet, as you’d need a separate sheet for each point in time. 1-dimensional automata can be displayed on a spreadsheet though, by simply using the top row to represent the cells you’re starting with and then iterating time down the sheet.

Screenshot of a spreadsheet on my phone, showing white and green cells in a cellular automaton pattern

CL-P’s spreadsheet models the famous Rule 30 cellular automaton, named for the specific rule it encodes. Stephen Wolfram, of Wolfram who brought you the maths programme Mathematica and many other cool maths things, named the set of Elementary Cellular Automata rules (0 to 31), each using a different pattern of outputs. The rules tell you how the cells in the row above determine the next row – each triple of three cells determines the value of the cells below, and each cell is allowed to be ‘on’ or ‘off’ (much like in Conway’s Game of Life, where the cells are ‘alive’ or ‘dead’). Since there are 8 possible combinations of on/off across 3 cells, the rule is encoded by knowing whether each of the 8 combinations results in the cell below being ‘on’ or ‘off’. These 8 on/off values determine a binary number less than 32, and the rule is named after the number given by the set of on/offs it uses, as 0/1 digits.

Of the 31 possible rule-sets Wolfram determined in this way, 30 is probably the most well-known, mainly because it’s the most interesting. $30 = 00011110_2$, which means if the number given by the three cells above as binary digits is 0,6,7 or 8 then the cell below should have a 0, and if not then it should have a 1.

Some of them end up dying a death pretty quickly (rule 0 is particularly boring – whatever you start with, everything in the second row is dead, and everything after that is dead forever). Rule 30 however, gives a pleasing pattern of shapes and even if you just start with a single ‘on’ cell in the top row, propagates a pattern across the whole sheet. Christian’s found a way to incorporate the statement of Rule 30 into a single formula:

=if(mod(4*[cell above left]+2*[cell above]+[cell above right]-3,7)<4,0,1)
Screenshot of a spreadsheet on my phone, showing white and green cells in a cellular automaton pattern
Having changed a couple of the cells in the top row from 1 to 0

This says, if the three cells above left to right are A, B and C, then if $4A + 2B + C – 3 $ (the binary number given by the numbers above, minus 3) is less than $4 \mod 7$ – so, if the number above is 0,6,7 or 8, then don’t colour the cell, and if it’s not then do colour the cell. Conditional formatting is used to make cells with a 1 in turn green, and cells with a 0 will remain white.

Christian’s sheet is here: CP’s Rule 30 spreadsheet

If you’d like to play with it, you can make a copy of it in your own Drive and edit there – we’ve left this one as view-only so you can always come back and get an undamaged version if you make a mess of things. Try starting with different combinations of 0/1 across the top, and make sure you give it a little time to calculate the rest of the sheet – it’s thinking pretty hard for a little phone!

Sock, Horror indeed

You might have seen our Aperiodical Round-Up post, which was posted recently despite languishing in the drafts folder for six years. It wasn’t alone in there, and we’ve found a few other posts which somehow didn’t get published at the time, which we’re planning to release any day now when we get a minute. Enjoy this classic nonsense formula post from circa April 2016.

In our constant quest to make sure people aren’t abusing maths too badly, we recently came across a new campaign from a certain corporate electronics giant, who have invented a washing machine with a little door in the door. A meta-door, if you will, so you can add extra items while a wash is going on.

Resources for learning probability and statistics

I was recently asked for some recommendations of resources for learning about probability and statistics, for someone without a strong mathematical background. I did a little digging, and have collated what I found here in case it’s useful to anyone else. Add your own suggestions in the comments!

From the mailbag – circles of circles

Katie’s dad needs a little maths advice – so he’s asked his daughter for some help, and she’s used it as an excuse to go on about maths for a while, as usual.

Dear The Aperiodical,

I’ve been working on a scale model of an underground mine, which will be lit using fibre optics – thin cylindrical strands of clear material, which can transmit light from a source at one end and output light at points all over the model.

All the strands will come together behind the scenes and be pointed at a light source inside a box – but if I have 24 strands with circular cross-section, each 1.5mm in diameter, how big a hole would I need to drill in the box for all the strands to pass through?

Photograph of mine model

Mathematical Things To Do

Image by congerdesign from Pixabay

If you find yourself at a loose end this month, want a break from focusing on work, or have younger mathematicians to entertain, here are some suggestions for online activities you can do/watch/attend. If you have any suggestions of your own, add them in the comments!

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