Mathematical beauty

Javier Irastorza:

Reading science books for the general public, you’ll often find physicists talking about elegance, beauty and words of the like describing laws or theories.
The Wikipedia has an entry for “Mathematical Beauty“. Another entry says “Many mathematicians talk about the elegance of mathematics, its intrinsic aesthetics and inner beauty. Simplicity and generality are valued. There is beauty in a simple and elegant proof […]”.
The Spanish journal El Pais is publishing each week a mathematical challenge to its readers to commemorate the 100th anniversary of the Spanish Royal Mathematics Society.
Last week’s challenge was to solve the sides of the different inner squares that compose the following rectangle, knowing that the red one has a side of 3.