The beautiful image that you see on the cover is a fractal, a recent entrant into the world of mathematics, dating from the 1980s in the work of the mathematician Benoit Mandelbrot, who himself coined the term ‘fractal’. Mandelbrot recognised that most objects found in nature cannot be modelled very well by the regular objects we encounter in Euclidean geometry (triangles, rectangles, circles, spheres…). Here are some famous quotes of his about fractals:

“A fractal is a mathematical set or concrete object that is irregular or fragmented at all scales…”

Essentially, the term denotes a set of points whose dimension can be regarded, in a denite and precise sense, as non-integral.