Selected answers:
Centralizers of pairs in monoids of self-maps
https://mathoverflow.net/questions/255448/a-question-about-subsets-of-plane/255478#255478 (or: how to apply the notion of commensurating action to a plane geometry question);
https://mathoverflow.net/questions/208645/divisors-of-p41-of-the-form-kp1/208652#208652
https://mathoverflow.net/questions/145615/do-constructible-sets-have-krull-dimension/146350#146350