IV) commutative, non-associative operations
For any natural number n, let denote the set of all operations on a set of that order. An operation ★ shall be called commutative iff and be called associative iff holds.
With the above defined, one may study . For n = 2, this set is nonempty for the first time, containing a manageable two elements, by name
However, based on the superexponential nature of , the sequence likely also grows rather quickly, OEIS only listing four members;
Based on this limited numerical evidence, I would suspect the commutative yet non-associative operations to be rather sparse, i.e.
Analysis source: operations.hs
(Non-)commutative and (non-)associative operations have also been studied nearly twenty years ago by Christian van den Bosch, author of OEIS sequence A079195. Unfortunately, their site appears to be down, which is where they hosted Closed binary operations on small sets (resource found on web.archive.org).
V) Arbitrary polynomial extremum difference
Let ε > 0 be an arbitrary distance, define . Then has two local maxima at -1 and 1, whose vertical distance is ε.
VI) Digit sum roots
It holds that .