Another resource I found in HN discussions: https://latypoff.com/tree-calculus-visualized/

A good point! From the “visual introduction” post mentioned elsewhere: Rules 1 and 2 seem arbitrary […], but behave analogous to the K and S operators of combinatory logic, which is sufficient to bootstrap λ-calculus. Rules 3a-c “triage” what happens next based on whether the argument tree is a leaf, stem or fork. This allows writing reflective programs.

See Barry’s post https://github.com/barry-jay-personal/blog/blob/main/2024-12... for more discussion.

This isn't a math thing[1], it's a theoretical computing model (ie instead of a Turing machine or lambda calculus, you can use this instead) that you might study as part of studying computation theory or other bits of theoretical computer science.

[1] or not pure maths anyway. It's applied maths like all computer science.