Triangle Lectures in Combinatorics (TLC)

These are a series of combinatorial workshops, held once per semester, each on a Saturday. The series began with its first meeting in spring 2010. They rotate among the universities in and around the Research Triangle. Participants come from numerous colleges and universities within a few hours drive, and some from even farther away. These workshops are funded by the National Science Foundation and the National Security Agency, in particular enabling us to bring in four exciting speakers to give one hour talks each time as well as funding travel expenses for participants.

TLC steering committee: Patricia Hersh (NCSU), Ezra Miller (Duke), Gabor Pataki (UNC Chapel Hill), Scott Provan (UNC Chapel Hill), Nathan Reading (NCSU), and Seth Sullivant (NCSU).

Next meeting: Saturday October 4, 2014, 9:15am -- 5pm.

Location: High Point University in High Point, North Carolina (one hour west of the Research Triangle by car).

Lecture Hall: 206 Cogdon Hall MAP

Speakers: Richard Ehrenborg (Kentucky), Robert Ghrist (Penn), Luis Serrano (U Quebec a Montreal), and Rekha Thomas (U. Washington).

Participant travel funding: We have funds available for travel and hotel for participants, especially for early career participants, thanks to grants from the NSF and the NSA. Much of this is restricted to U.S. citizens and permanent residents, and what is available to others still requires that the participants be employed at a U.S. university. To apply for funding fill out the form here: Travel Funding Request Form If you have questions, please contact Nathan Reading (

Saturday Triangle Lectures in Combinatorics Schedule:

9:15-10am, coffee and bagels
10-11am, Rob Ghrist, TBA
11-11:30am, coffee break
11:30am-12:30pm, Luis Serrano, The immaculate basis of the non-commutative symmetric functions
12:30-2:30pm, lunch break
2:30-3:30pm, Rekha Thomas, The Euclidean distance degree of an algebraic variety
3:30-4pm, coffee break
4-5pm, Richard Ehrenborg, Euler enumeration
6pm, somewhat informal conference dinner at Sumela

Logistical information: The nearest airport is Greensboro/High Point (GSO), 20 minutes drive away. This is also known as the Piedmont Triad Airport. Within 90 minutes drive are both Raleigh-Durham (RDU) Airport and Charlotte (CLT) Airport. A cab from GSO costs about $30 each way. Hotels are about 1.5 to 3 miles from campus. Thus, a car rental may make sense for anyone flying to the meeting.

A block of hotel rooms has been reserved at the Courtyard Marriott High Point (336-882-3600), which is 1.7 miles from campus. The rooms are blocked off under "The TLC" until September 19th. However, if rooms are still available, they will honor our group rate of $119 per night (+tax) after this date. (The room includes two queen beds, wireless internet, and either a hot breakfast sandwich or two continental breakfast items, plus beverage, for 2 adults). There are many other local options including: -High Point Plaza , 1 mile from campus (more downtown). 336-889-8888 -Crestwood Suites, about 2.7 miles from campus (more economical). 336-886-5665.

Preregistered Participants (so far):

Ed Allen, Wake Forest
Justin Allman, Virgina Tech
Jennifer Anderson, Marshall University
Yue Cai, U Kentucky
Robert Davis, U Kentucky
Rafael S. Gonzalez D'Leon, U Kentucky
Vivek Dhand, Charlottesville, VA
Richard Ehrenborg, U Kentucky
Norman Fox, U Kentucky
Rob Ghrist, U Penn
Alex Happ, U Kentucky
Qijun He, Clemson
Wesley Hough, U Kentucky
Florian Kohl, U Kentucky
Sarah Mason, Wake Forest
Marie Meyer, U Kentucky
John Mosley, U Kentucky
Vidit Nanda, U Penn
Yusra Naqvi, Rutgers
Sarah Nelson, U Kentucky
Elizabeth Niese, Marshall University
Sarah Orchard, U Kentucky
Lindsay Piechnik, High Point
Radmila Sazdanovic, NCSU
Luis Serrano, UQAM
Cliff Smyth, UNC Greensboro
Seth Sullivant, NCSU
Rekha Thomas, U Washington
Laurie Zack, High Point

Talk titles and abstracts (received so far):

Richard Ehrenborg (U Kentucky)

Title: Euler enumeration

Abstract: The flag vector contains all the face incidence data of a polytope, and in the poset setting, the chain enumerative data. It is a classical result due to Bayer and Klapper that for face lattices of polytopes, and more generally, Eulerian graded posets, the flag vector can be written as a cd-index, a non-commutative polynomial which removes all the linear redundancies among the flag vector entries. This result holds for regular CW complexes. We relax the regularity conditions to show the cd-index exists for manifolds whose boundary has a Whitney stratification. The setting of Whitney stratifications allows us to give shorter proofs of identities involving the cd-index and opens inequality questions for manifolds. This is joint work with Mark Goresky and Margaret Readdy.

Luis Serrano (UQaM)

Title: The immaculate basis of the non-commutative symmetric functions

Abstract: We introduce a new basis of the non-commutative symmetric functions whose elements have Schur functions as their commutative images. Dually, we build a basis of the quasi-symmetric functions which expand positively in the fundamental quasi-symmetric functions and decompose Schur functions according to a signed combinatorial formula. These bases have many interesting properties similar to those of the Schur basis, and we will outline a few of them.

Rekha Thomas (U Washington)

Title: The Euclidean distance degree of an algebraic variety

Abstract: It is a common problem in optimization to minimize the Euclidean distance from a given data point u to some set X. In this talk I will consider the situation in which X is an algebraic variety, a common situation in practice. The number of critical points of the objective function on X is called the Euclidean distance degree of X, and is an intrinsic measure of the complexity of this polynomial optimization problem. Using algebraic geometry we obtain formulas and methods to calculate this degree in many situations. I will explain the results and illustrate the formulas that can be obtained in several situations ranging from matrix analysis to control theory to computer vision. Joint work with Jan Draisma, Emil Horobet, Giorgio Ottaviani and Bernd Sturmfels.

Organizing Committee: Lindsay Piechnik (chair, High Point), Ed Allen (Wake Forest), Sarah Mason (Wake Forest), Seth Sullivant (NCSU), and Laurie Zack (High Point)

Past meetings:

Ninth meeting: February 22, 2014 at UNC-Chapel Hill.
Speakers: Shankar Bhamidi (UNC-Chapel Hill), Anders Buch (Rutgers), Pablo Parrilo (MIT), Eva Tardos (Cornell)
Organizing committee: Gabor Pataki (UNC-Chapel Hill), Lindsay Piechnik (High Point University), Scott Provan (UNC-Chapel Hill), Richard Rimanyi (UNC-Chapel Hill), Jack Snoeyink (UNC-Chapel Hill)
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Eighth meeting: September 21, 2013 at NCSU.
Speakers: George Andrews (Penn State), Matt Beck (San Francisco State University), Robin Pemantle (University of Pennsylvania), and Victoria Powers (Emory University)
Organizing committee: Michael Singer (NCSU), Carla Savage (NCSU), and Seth Sullivant (NCSU)
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Seventh meeting: February 9, 2013 at Wake Forest University.
Speakers: Louis Billera (Cornell), Rod Canfield (University of Georgia), Matthew Kahle (Ohio State University), Michelle Wachs (University of Miami)
Organizing committee: Sarah Mason (chair, Wake Forest University), Ed Allen (Wake Forest University), Alex Fink (NCSU), Patricia Hersh (NCSU)
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Sixth meeting: September 22, 2012 at NCSU.
Speakers: Allen Knutson (Cornell), Vin de Silva (Pomona College), Richard Stanley (MIT), Lauren Williams (UC Berkeley).
Organizing committee: Alex Fink (NCSU), Patricia Hersh (NCSU), Carla Savage (NCSU).
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Fifth meeting: February 11, 2012 at Duke University.
Speakers: Alex Fink (NCSU), Sergey Fomin (Michigan), Nets Katz (Indiana University), Isabella Novik (University of Washington).
Organizing Committee: Christine Berkesch (Duke), Sonja Mapes (Duke), Ezra Miller (Duke).
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Fourth meeting: November 5, 2011 at UNC Chapel Hill.
Speakers: Thomas Lam (Michigan), Jesus De Loera (UC Davis), Ezra Miller (Duke), Doron Zeilberger (Rutgers)
Organizing committee: Prakash Belkale (UNC Chapel Hill), Gabor Pataki (UNC Chapel Hill), Robert Proctor (UNC Chapel Hill), Scott Provan (UNC Chapel Hill), Richard Rimanyi (UNC Chapel Hill).
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Third meeting: April 9, 2011 at NCSU.
Speakers: Prakash Belkale (UNC Chapel Hill), Vic Reiner (University of Minnesota), John Stembridge (University of Michigan), Stephanie van Willigenburg (UBC).
Organizing committee: Hoda Bidkhori (NCSU), Alex Fink (NCSU), Patricia Hersh (NCSU), Carla Savage (NCSU).
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Second meeting: September 25, 2010 at Duke.
Speakers: Alexander Barvinok (University of Michigan), Anne Shiu (Duke), Sami Assaf (MIT), Persi Diaconis (Stanford).
Organizing committee: Patricia Hersh (NCSU), Sonja Mapes (Duke), Ezra Miller (Duke).
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First meeting: February 6, 2010 at NCSU.
Speakers: Carla Savage (NCSU), Bernd Sturmfels (UC Berkeley), Ed Swartz (Cornell), Laszlo Szekely (University of South Carolina).
Organizing committee: Patricia Hersh (NCSU), Ezra Miller (Duke), Scott Provan (UNC) Nathan Reading (NCSU).
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