Combinatorics Seminars

Spring 2018

Speaker: Lou Shapiro, Howard University
Date and time: Tuesday, January 30, 1-2pm
Place: Rome 771
Title: Combinatorics and the Quadratic Formula
Abstract: We explore the connection between the quadratic formula and the Catalan numbers. We start with a piece of folklore, then move to examples including the drunk on the cliff. Then combinatorial interpretations, the Riordan group, and involutions in the Riordan group.

Speaker: Jim Lawrence, George Mason University
Date and time: Tuesday, January 23, 1-2pm
Place: Rome 771
Title: Interval posets, parity representations, binary partitions, and antiprisms
Abstract: Given a poset, one obtains another poset by considering the collection of intervals of the first, partially ordered by inclusion.  (There are various possibilities, depending, for instance, upon whether one considers the empty set as being an ``interval.'')  This construction has found use in the study of convex polytopes and other places.  We describe a new method of representation of posets by utilizing certain geometric complexes in R^d having vertices in Z^d.  The striking feature of this method of representation is that taking the  interval poset corresponds to dilation by a factor of 2 of the geometric complex.  We explore connections with the integer partitions of powers of 2 into powers of 2.