"The Yang-Baxter equation is a simple equation that can be represented by a picture that a two-year-old can draw," says Robert Weston of Heriot-Watt University in Edinburgh, UK.

Like the Euler-Lagrange equation, it looks simple but has profound implications for many areas of mathematics and physics.

These include how waves behave in shallow water, the interaction of subatomic particles, the mathematical theory of knots, and string theory.

"You might imagine it as being at the centre of a spider's web," says Weston. "On the strands of that web you can find numerous topics in which it plays a fundamental role."

The equation does not look like it has anything to do with these subjects, and that is part of its appeal for Weston.

"I am surprised and sometimes bewildered that everyday, concrete, physical systems are best described using some of the most abstract mathematical structures developed in the last 50 years, and I am astonished that people have been able to figure this out."

Read more: What is the most beautiful equation?