A neat way of separating a deck of playing cards into red and black piles is the one-two trick: move the pack from one hand to the other, taking clumps of one colour or two colours alternately.
Takes at most four runs through to completely separate the colours in a 52-card deck. (Why?)
Mad Abel is a card game for wo or more players which involves multiplication in the finite group of your choice: http://read.somethingorotherwhatever.com/entry/Mccarthy2006
A Mastodon instance for maths people. The kind of people who make \(\pi z^2 \times a\) jokes.
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