There has been a pause in my blogging. I have been engrossed in other work. The pause may last for a while longer. So here is a little puzzle, as an entr'acte.
According to Wikipedia, Elizabeth of York, wife of Henry VII, is the most recent common ancestor of all English monarchs. I have not checked this claim, but let us assume that it is correct. The puzzle, which we would need to solve before checking the correctness of the claim, is as follows.
How should we define "most recent common ancestor" in this context, so that a determinate person, or a determinate couple, is picked out, and so that it is interesting that this person, or this couple, is picked out?
The condition of interest would be failed if our definition led us to pick out George VI, or even if it led us to pick out George V, or Victoria. We want some sense of "furthest back most recent". But then, we must not allow Elizabeth of York's mother, Elizabeth Woodville, to displace her.
Technical terms from mathematics may be used freely.
Blogspot does not, so far as I know, support LaTeX math environment, so please feel free either to cut and paste logical symbols from elsewhere, or to use E for the existential quantifier, V for the universal quantifier, - for negation, and > for implication.