Robuta

https://math101.guru/en/tag/collatz-conjecture/
collatz conjecturethe greatmysteriesmath
https://kottke.org/16/08/the-infamous-collatz-conjecture
For a recent episode of Numberphile, David Eisenbud explains the Collatz Conjecture, a math problem that is very easy to understand but has an entire book dev
the infamouscollatz conjecture