Lose yourself in an island of mathematics

If this existed 200 years ago, kings and queens would sacrifice half their subjects to be able to play with it. Philosophers and artists and mathematicians would lose their freaking minds. Now, we buy it for 99 cents. It really does give you the sense, which is true by the way, that math is a vast continent and you can easily amuse yourself on any one of its beautiful, idiosyncratic islands.

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