New York Combinatorics Seminar

This seminar is sponsored by the CUNY Graduate Center's Math Department and Computer Science Department. It covers a wide range of topics in combinatorics and its applications.  Talks are posted on our YouTube channel https://youtube.com/@NYCombinatorics and catalogued on the page Previous Semesters.

Time: Fridays 12:00 pm:  1:00 pm ET (Eastern Time)
Location: The CUNY Graduate Center is located at 365 Fifth Avenue (at the corner of 34th Street), New York. It can be easily reached by subway using the B,D,F,N,Q,R, or 6 train.

Seminar Co-Organizers (alphabetically):  Kira Adaricheva (Hofstra University) Deepak Bal (Montclair State University) Nadia Benakli (City Tech) Jonathan Cutler (Montclair State) Ezra Halleck (City Tech), Sandra Kingan (Graduate Center & Brooklyn College), Joseph Malkevitch (Graduate Center & York College), Kerry Ojakian (BCC), Megan Owen (Graduate Center & Lehman College),  Jinyoung Park (New York University), Anna Pun (Graduate Center & Baruch College), Eric Ramos (Stevens Institute of Technology), (Abigail Raz (Cooper Union), Eric Rowland (Hofstra University), Mingxian Zhong (Graduate Center & Lehman College).

Lead Organizer:
2025 - 2026: Deepak Bal
2024 - 2025: Anna Pun
2011 - 2024: Sandra Kingan

FALL 2025 TALKS

Room for In-person talks: Graduate Center 9116

Zoom link: https://montclair.zoom.us/j/93937899930?pwd=t3CoLts4pfRXcmhpUsUI6fIys9nv0p.1
Meeting ID: 93937899930     Passcode nycs

Sep 12:  Christopher Hanusa (Queens College) (in-person)

Title:  Mathematical Illustration through a combinatorial lens 

Abstract: In this talk, I will share my decade-long journey as a mathematical artist and jewelry maker and how it intertwines with my mathematics research in combinatorics. I will describe the mathematical ideas that motivate the art pieces, many of which are combinatorial in nature. Attendees will experience first-hand a variety of mathematical sculptures and have a hands-on experience with interactive multimedia pieces: Fractal Emergence and Experiential Braid Relations. The talk with conclude with ways in which mathematicians can participate in the broader mathematical art community. All members of the CUNY mathematical community are welcome to attend. 
 

Sep 19:  No Talk

Sep 26: Joshua Birns (Stevens University) (in-person)

Title: Graph Isomorphism Testing - Recent Advances

Abstract: Graph isomorphism is one of the most fundamental problems of study in combinatorics. It arises of natural mathematical intuition and has important practical ramifications. In this talk we will survey important developments of the problem, as well as discuss why purely combinatorial approaches to this problem fail. 

Oct 3: Amanda Redlich (UMass Lowell)  (in person)

Title: $C_4$ is a square in more ways than one: Square roots in the graph gluing algebra

Abstract: Some newly-developed proof techniques  in subgraph homomorphism density (see e.g. Blekherman, Raymond, Singh, Thomas) motivated interest in the "gluing algebra” of graphs, and in particular which graphs under that algebra have non-trivial square roots.  However, this question is of independent interest: "square” graphs have a specific type of symmetry, and are surprisingly hard to identify.  In this talk I will introduce some square-rooting techniques and describe open questions.   I will also prove that $C_4$ is a square.  Research discussed is joint with Karen Collins, David Galvin, Christine Kelley, Emily McMillon, Annie Raymond.

Oct 10: Ian Seong (Williams College)  (in-person)

Title: The nucleus of the Grassmann graph $J_q(N,D)$

 Abstract: Consider a permutation group on three elements - this group has two irreducible representations on a 3D vector space over the real numbers. This is an example of a group action on a vector space. We now turn our attention to the action of an adjacency algebra (and its relatives) of a distance-regular graph G acting on a vector space V, where the basis vectors are characterized by the vertices. In this talk we discuss the action of a Terwilliger algebra T of G and the irreducible T-modules in V. In particular, these modules have an endpoint r, dual endpoint t, and diameter d. The displacement of an irreducible T-module is defined to be the integer r+t-D+d, where D is the diameter of the graph G. The nucleus is defined to be the span of irreducible T-modules that have displacement 0. The goal of this talk is to describe the nucleus of a Grassmann graph, a well-known family of distance-regular graphs.

Oct 17:  Rafael S. González D'León (Loyola University Chicago) (in-person)

Title: On Whitney numbers of the first and second kind (or is it the other way around?)

Abstract: The Whitney numbers of the first and second kind are a pair of poset invariants that appear in surprising places throughout mathematics. One of the most interesting appearances of these numbers is as the coefficients of the chromatic polynomial of a graph. They also appear as counting regions in the complement of a real hyperplane arrangement. In this talk, I will introduce these concepts and will share a very curious phenomenon: sometimes the Whitney numbers of the first and second kind of a poset are exactly the Whitney numbers of the second and first kind of a different poset. To find examples of this phenomenon we rely on the poset techniques of edge labelings and quotient posets. The results and open questions that will be presented are joint work with Josh Hallam, Yeison Quiceno, Andrés Molina, and José Samper.

Oct 24: J B Nation (University of Hawaii) (virtual)

Title: Plane geometries from vector norms and the Pythagorean Theorem

Abstract: A strongly normed plane is a geometry (R^2, d) where R denotes the real numbers and the distance function "d" is derived from a vector norm "f", that is, 

d(u,v) = f(u - v). As long as the unit "circle" f(u)=1 is strictly convex, such a plane has exactly the same points and lines as the standard Euclidean plane. Distances and 

angles, of course, are not the same.  If "f" is a positive definite quadratic form, then we get an inner product space isomorphic to the standard plane. Other strongly normed planes, however, can behave quite differently with respect to right angles and orthogonality. 

Oct 31: Daniel Johnston (Trinity College) (in-person)

Title: The Saturation Spectrum of Berge Stars

Abstract: Given two $k$-uniform hypergraphs $F$ and $H$, we say $H$ is $F$-saturated if $H$ does not contain a copy of $F$, but $H+e$ does for any edge $e\in E(\overline{H})$. The $n$-vertex saturation spectrum of a hypergraph $F$ is the set of all integers $m$ such that there exists an $n$-vertex $m$-edge $F$-saturated hypergraph.  Given a graph $G$ and a hypergraph $H$ embedded on the same vertex set, we say $H$ is a Berge-$G$ if there exists a bijection $\phi:E(G)\to E(H)$ such that $e\subseteq \phi(e)$ for all $e\in E(G)$.  In this talk, we look at the saturation spectrum for $3$-uniform Berge stars. This is joint work with N. Bushaw, S. English, E. Heath, and P. Rombach.

Fri Nov 7: 81st Graph Theory Day of New York (GTD81 2025)
Location:  Montclair State University
Keynote Speakers:   Garth Isaak (Lehigh University) and Katie Haymaker (Villanova University)
Please encourage your students to present posters in the poster session.

Nov 14:

Nov 21: Jonathan Cutler (Montclair State University) (in-person)

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Previous Speakers:

Spring 2025
Fall 2024
Spring 2024
Fall 2023
Spring 2023
Fall 2022
Spring 2022
Fall 2021
Spring 2021
Fall 2020
Spring 2020
Fall 2019
Spring 2019
Fall 2018
Spring and Summer 2018
Fall 2017
Spring 2017
Fall 2016
Spring 2016
Fall 2015
Spring 2015
Fall 2014
Spring 2014
Fall 2013
Spring 2013
Fall 2012
Spring 2012
Fall 2011
Spring 2011
Previous Talks hosted by Janos Pach

Previous Organizers:
Christopher Hanusa (2011 - 2015)