Recently Quanta magazine came out with an article explaining modular forms:
• Jordana Cepelewicz, Behold modular forms, the ‘fifth fundamental operation’ of math, Quanta, 23 September 2023.
This is a companion discussion topic for the original entry at https://johncarlosbaez.wordpress.com/2023/09/23/the-moduli-space-of-acute-triangles/