“Two, four, six, eight; bog in, don’t wait”. As children, we learn numbers can either be even or odd. And there are many ways to categorise numbers as even or odd. We may memorise the rule that ...

There is a long, beautiful history of thinking about numbers and shapes as two aspects of the same idea. Square numbers are often described using arithmetic: 1 = 1 x 1 = 1, 2 = 2 x 2 = 4, 3 = 3 x 3 = ...