This Week's Finds 14: the 3-strand braid group

The 3-strand braid group has striking connections to the group SL(2,ℤ) of invertible 2x2 matrices with integer entries, the Lorentz group from special relativity, modular forms (famous in number theory), and the trefoil knot. They all fit together in a neat package, which I explain here.
This is one of a series of lectures at the University of Edinburgh on topics drawn from my column This Week's Finds. This is the fourth lecture of 2023. For other lectures go here:
math.ucr.edu/ho...