Cool monoid alert!
The Kuratowski monoid represents all possible combinations of complement (c) and closure (k) operations on subsets of a topological space. (Note that the interior operation can be given as the composition ckc.) Only 14 possible operations exist.
See:
Kuratowski's closure-complement problem - Wikipedia
This diagram is from the paper:
Schwiebert, Ryan (2017). "The radical-annihilator monoid of a ring". Communications in Algebra. 45 (4): 1601–1617.
