Logic, Category theory
Set theory
https://en.wikipedia.org/wiki/Goodstein%27s_theorem
- Shows that the provability of a statement may depend on how powerful is the notation supported by the axioms.
- Gives clues on how the limitations of Goedel's Incompleteness theorem https://en.wikipedia.org/wiki/G%C3%B6del%27s_incompleteness_theorems might be overcome, for example, by a growing set of axioms, potentially infinite, by which the notation would become ever more powerful, thus becoming more complete.