28.2 “At Most”
Learning to translate “at least” in the previous section was hopefully pretty intuitive. “At most” is a bit messier.
First notice that saying “There are at most 2 dogs” doesn’t tell us that any dogs exist. “At most 2” means 2 or fewer.
“There are at most two dogs” is true if there are 2, 1 or 0 dogs. That tells us we don’t want an existential quantifiers.
Instead, what we’ll say in FOL is: If variables x, y and z all pick out dogs, then two or more of those variables need to be picking out the same dog.
It looks like this:
∀x∀y∀z((D(x)&D(y)&D(z))→(x=y v x=z v y=z))
Recall that generalizations in FOL can be vacuously true, so this does what we wanted: it doesn’t entail the existence of any dogs.
Notice that the number of equalities you need in the consequent of the arrow also depends on how number n we are saying there are “at most n” of.
For “at most 1”, we need 1 equality. For “at most 2”, we need 3. The progression works by triangular numbers here as well.