Combinatorial Potlatch 2018
Simon Fraser University
Saturday, November 17, 2018

About the Combinatorial Potlatch

The Combinatorial Potlatch is an irregularly scheduled, floating, one-day conference. It has been held for many years at various locations around Puget Sound and southern British Columbia, and is an opportunity for combinatorialists in the region to gather informally for a day of invited talks and conversation. While most who attend work in, or near, the Puget Sound basin, all are welcome.  Typically there are three talks given by speakers who are visiting or new to the area, along with breaks for coffee and lunch. Many participants remain for dinner at a local restaurant or pub.

The American Heritage Dictionary defines "potlatch" as: A ceremonial feast among certain Native American peoples of the northwest Pacific coast, as in celebration of a marriage or an accession, at which the host distributes gifts according to each guest's rank or status. Between rival groups the potlatch could involve extravagant or competitive giving and destruction by the host of valued items as a display of superior wealth. [Chinook Jargon, from Nootka p'achitl, to make a potlatch gift.]

This Fall's Potlatch is being hosted by the Department of Mathematics at Simon Fraser University at their downtown campus in Vancouver, BC on Saturday, November 17, 2018.

Significant funding is being provided by the Pacific Institute for the Mathematical Sciences and the SFU Department of Mathematics. Their support is gratefully acknowledged.

More info, including a history and links to previous Potlatches, is at The Combinatorial Potlatch Home Page.


All talks will be held in the Fletcher Challenge Theatre at SFU Harbour Centre, with registration and breaks nearby. See the Getting There section for exact locations and directions.

A tentative schedule follows.

  • 10:00 AM Registration, Bagels and Coffee
  • 11:00 AM Talk: Shahla Nasserasr
  • 12:00 PM Lunch at Irish Heather
  •   2:30 PM Talk: Orit Raz
  •   3:00 PM Cookies, Coffee and Cokes
  •   3:30 PM Talk: Fan Chung
  •   5:00 PM Happy Hour, Dinner at Steamworks Brewpub

Talks and Abstracts

Shahla Nasserasr, Brandon University

The Inverse Eigenvalue Problem: Distinct Eigenvalues

Let $S(G)$ be the set of real symmetric matrices whose nonzero off-diagonal entries correspond to the edges of the graph $G$. The inverse eigenvalue problem is to determine the set of all possible eigenvalues of matrices in $S(G)$. In this talk, we will focus on $q(G)$, the minimum number of distinct eigenvalues of a matrix in $S(G)$. We will survey some results about the parameter $q(G)$, and show that the graphs with $q(G) = 2$ cannot be characterized in terms of a finite collection of forbidden structures.

Orit Raz, University of British Columbia

Every Embedding of a Dense Graph has a Rigid Subset

While the problem of determining whether an embedding of a graph $G$ in ${\mathbb R}^2$ is infinitesimally rigid is well understood, specifying whether a given embedding of $G$ is rigid or not is still a hard task that usually requires ad hoc arguments. In the talk I will discuss a recent result (joint with Jozsef Solymosi), where we show that every embedding of a sufficiently dense graph has a rigid subset. The proof uses a reduction of the original rigidity problem to a question about line configurations in ${\mathbb R}^3$.

Fan Chung, University of California, San Diego

Geometric Aspects in Spectral Graph Theory

We will briefly survey some recent developments and open problems in spectral graph theory. In particular, we will discuss the geometric implications of the spectral gaps including the expansion of vertex/edge neighborhoods and the edge stretches in eigenfunctions using the curvature of graphs. We consider variations of Ramanujan graphs and the related spectral bounds for graphs with general degree distributions.


The Combinatorial Potlatch has no permanent organization and no budget. And we like it that way. Consequently, there are no registration fees because we wouldn't know what to do with them. You are on your own for meals and lodging, and the sponsoring institutions provide facilities, food for the breaks and sometimes a little support for speakers' travel. So expressions of appreciation to the speakers and the hosts are preferred and especially encouraged. Thanks.

Getting There

All talks will be held in the Fletcher Challenge Theatre at SFU Harbour Centre, 515 West Hastings Street (at the corner of Hastings and Richards), with registration and breaks nearby. Directions are here. The closest parking lot is at 400 West Cordova Street.


YWCA Hotel
733 Beatty Street, Vancouver. Both male and female guests are welcome. Located 900m from SFU Harbour Centre (about 12 minutes to walk). Range of private room types accommodating between 1 and 4 people, with hallway, shared, or private bathroom facilities. Nightly rates from \$84 (single bed with hallway bath) to \$136.50 (2 double beds with private bath, additional \$15.75 for a third and \$15.75 for a fourth occupant), plus tax (currently 12.5%). Reduction for seniors (60+ years old) and students (valid student ID required at check in).
Delta Vancouver Suites
550 West Hastings Street, Vancouver (across the road from SFU Harbour Centre). Discounted rates for the nights of Friday 16 and Saturday 17 November: \$204/night for single or double occupancy, additional \$30/night for a third and \$30/night for a fourth occupant, plus tax (currently 17.5%), available through this link.

Dining and Happy Hour

You are encouraged to join other conference participants at the various meals and other events we have planned for the day. Details are below.

Lunch: Irish Heather, 210 Carrall Street, Gastown

Happy Hour, Dinner: Steamworks Brewpub, 375 Water Street, Gastown


  • Nancy Ann Neudauer, Pacific University, nancy (at) pacificu (dot) edu, Program Chair
  • Amites Sarkar, Western Washington University, amites (dot) sarkar (at) wwu (dot) edu, Communications Chair
  • Jonathan Jedwab, Simon Fraser University, jed (at) sfu (dot) ca, Local Arrangements Chair
Last updated: October 29, 2018