Since none was published, I tried to "clean room reverse engineer" it: give one Claude Fable the result and ask it to generate a writeup of how to arrive at it without any spoilers, then ask a second Fable to follow the write up, if it's too easy remove details, if it's too hard add hints. I could have simplified further, I stopped because it's time to sleep, and I'm sharing because it gave me some intuition for what's going on.

This is what a successful run looks like (sadly Anthropic hide Chains of Thought):

https://claude.ai/share/80526d56-1c23-407d-8f5c-59a704221454

## Intuition

This is my understanding of the interesting ideas (probably too summarized for Fable to consistently get it without a good harness; take with a grain of salt, I'm not a mathematician; later there is a full prompt that was tested and contains everything needed):

* No Bass–Connell–Wright or Drużkowski normal forms (those are reparametrizations that feel natural in this search but they turn low-complexity examples into high-complexity by trading off degree vs dimension, and the counterexample is low complexity)

* Look in C^3, not C^2, C^2 is probably not interesting enough

* Look for a 3:1 cover, not 2:1, there's some result due to Euler (as always) that shows 2:1 will not work

* Look for a composition of two functions (a ratio of polynomials and a shear) that have Jacobian determinants `x` and `c/x` everywhere, except at `x=0` (do the standard trick for not defining at a hole and shoving all the problems into that one hole, that's where the `1 + xy` comes from)

## Prompt for reproducing CoT

Here: https://gist.github.com/SonOfLilit/8882a145048ba260b160568ba...