December 6
Chris Laskowski,
University of Maryland
Counting siblings
Two countable structures are siblings if each is embeddable into the other (e.g., any two countable non-scattered linear orders are siblings). Clearly if M and N are siblings, they have the same finite substructures, hence have the same universal theories. We characterize the universal theories in a finite, relational language that have a countable model with $2^{\aleph_0}$ siblings. This is joint work with Sam Braunfeld.