Organizing Committee
 Susanna Fishel
Arizona State University  Pamela E. Harris
Williams College  Rosa Orellana
Dartmouth College  Stephanie van Willigenburg
University of British Columbia
Abstract
The Women in Algebraic Combinatorics Research Community will bring together researchers at all stages of their careers in algebraic combinatorics, from both research and teachingfocused institutions, to work in groups of 46, each directed by a leading mathematician. The goals of this program are: to advance the frontiers of cuttingedge algebraic combinatorics, including through explicit computations and experimentation, and to strengthen the community of women working in algebraic combinatorics.
Successful applicants will be assigned to a group based on their research interests. The groups will work on open problems in algebraic combinatorics and closely related areas, including representation theory, special functions, and discrete geometry. Several of the proposed projects will extensively involve experimentation and computation, which will increase the likelihood that concrete progress is made over the course of the initial workshop and following 6 months, and provide useful training in computational mathematics.
In their personal statements, applicants should rank in order their top three choices of projects. They should also address their familiarity with the suggested prerequisites. Applicants are expected to attend the opening and closing workshops and meet regularly with their research group for the 6 months in between.
To be considered for this workshop please apply by July 5, 2021.
Confirmed Speakers & Participants
Talks will be presented virtually or inperson as indicated in the schedule below.
 Speaker
 Poster Presenter
 Attendee
 Virtual Attendee

Ying A Pun
University of Virginia

Olga Azenhas
University of Coimbra

Margaret Bayer
University of Kansas

Carolina Benedetti V.
Universidad de los Andes

Georgia Benkart
University of WisconsinMadison

Sarah Brauner
University of Minnesota

Samantha Dahlberg
Aurora University

Galen DorpalenBarry
University of Minnesota

Jennifer Elder
Rockhurst University

Nursel Erey
Gebze Technical University

Ilse Fischer
Universität Wien

Susanna Fishel
Arizona State University

Holley Friedlander
Dickinson College

Emily Gunawan
University of Oklahoma

Pamela E. Harris
Williams College

Selvi Kara
University of Utah

Sandra Kingan
Brooklyn College, CUNY

Caroline Klivans
Brown University

Nadia Lafreniere
Dartmouth College

Elizabeth Manosalva
Universidad de Talca

Erin McNicholas
Willamette University

Shaheen Nazir
Lahore University of Management Sciences

Elizabeth Niese
Marshall University

Isabella Novik
University of Washington

Kathryn Nyman
Willamette University

Rosa Orellana
Dartmouth College

Jianping Pan
North Carolina State University

Greta Panova
University of southern California

Casey Pinckney
University of Maine

Heather Russell
University of Richmond

Anne Schilling
UC Davis

Lisa Schneider
Salisbury University

Sara Solhjem
Minnesota State University Moorhead

Shraddha Srivastava
Uppsala University

Jessica Striker
North Dakota State University

Sheila Sundaram
Pierrepont School

Mojdeh Tarighat
University of Virginia

Bridget Tenner
DePaul University

Jacinta Torres
Institute of Mathematics of the Jagiellonian University

Julianna Tymoczko
Smith College

Stephanie van Willigenburg
University of British Columbia

Julianne Vega
Kennesaw State University

Shiyun Wang
University of Southern California

Amanda Welch
Eastern Illinois University

Huafei Yan
Texas A&M University

Chenchen Zhao
University of Southern California

Hailun Zheng
University of Copenhagen
Workshop Schedule
Thursday, August 5, 2021

9:45  10:00 am EDTWelcomeVirtual
 Brendan Hassett, ICERM/Brown University

10:00  10:20 am EDTOpening RemarksVirtual
 Susanna Fishel, Arizona State University
 Pamela E. Harris, Williams College
 Rosa Orellana, Dartmouth College
 Stephanie van Willigenburg, University of British Columbia

11:10  11:30 am EDTBreakCoffee Break  Virtual

11:30  11:50 am EDTProject # 3 Introduction  Perspectives on PermutationsVirtual
 Speaker
 Bridget Tenner, DePaul University
 Session Chair
 Pamela E. Harris, Williams College (Virtual)

11:50 am  1:00 pm EDTLunch/Free TimeVirtual

1:00  1:20 pm EDTProject # 1 Introduction  Investigating Convex Union Representable ComplexesVirtual
 Speaker
 Isabella Novik, University of Washington
 Session Chair
 Stephanie van Willigenburg, University of British Columbia (Virtual)

2:00  2:20 pm EDTBreakCoffee Break  Virtual

3:00  4:00 pm EDTProject Group MeetingsGroup Work  Virtual

4:00  5:00 pm EDTGathertown ReceptionReception  Virtual
Friday, August 6, 2021

10:00  10:20 am EDTProject # 7 Introduction  Dynamical Algebraic CombinatoricsVirtual
 Speaker
 Jessica Striker, North Dakota State University
 Session Chair
 Rosa Orellana, Dartmouth College (Virtual)
Abstract
https://share.cocalc.com/share/46710ac1d93d912fc55796f3cc6e2acfd31831d3/Sage%20Demo%20worksheets/ICERM_research_community_Striker.sagews?viewer=share

10:30  10:50 am EDTProject # 8 Introduction  Higher Dimensional ChipFiringVirtual
 Speaker
 Caroline Klivans, Brown University
 Session Chair
 Rosa Orellana, Dartmouth College (Virtual)

11:00  11:20 am EDTBreakCoffee Break  Virtual

11:20  11:40 am EDTProject # 9 Introductions  McKay Matrices for Super Objects and Connections with CharactersVirtual
 Speaker
 Georgia Benkart, University of WisconsinMadison
 Session Chair
 Rosa Orellana, Dartmouth College (Virtual)

11:50 am  1:00 pm EDTLunch/Free TimeVirtual

2:00  2:20 pm EDTBreakCoffee Break  Virtual

2:30  4:30 pm EDTProject Group MeetingsGroup Work  Virtual

4:30  5:00 pm EDTClosing RemarksVirtual
 Susanna Fishel, Arizona State University
 Pamela E. Harris, Williams College
 Rosa Orellana, Dartmouth College
 Stephanie van Willigenburg, University of British Columbia
Thursday, February 10, 2022

9:15  9:45 am ESTCheck In11th Floor Collaborative Space

9:45  10:00 am ESTWelcome11th Floor Lecture Hall
 Brendan Hassett, ICERM/Brown University

10:00  10:20 am ESTOpening Remarks11th Floor Lecture Hall
 Susanna Fishel, Arizona State University
 Pamela E. Harris, Williams College
 Rosa Orellana, Dartmouth College
 Stephanie van Willigenburg, University of British Columbia

10:20  10:50 am ESTProject 4: Alternating Sign Matrices and Plane Partitions11th Floor Lecture Hall
 Speakers
 Samantha Dahlberg, Aurora University
 Ilse Fischer, Universität Wien
 Holley Friedlander, Dickinson College
 Kathryn Nyman, Willamette University
 Greta Panova, University of southern California
 Session Chair
 Susanna Fishel, Arizona State University (Virtual)

11:00  11:30 am ESTProject 7: Dynamical Algebraic Combinatorics11th Floor Lecture Hall
 Speakers
 Jennifer Elder, Rockhurst University
 Nadia Lafreniere, Dartmouth College
 Erin McNicholas, Willamette University
 Jessica Striker, North Dakota State University
 Amanda Welch, Eastern Illinois University
 Session Chairs
 Samantha Dahlberg, Aurora University
 Susanna Fishel, Arizona State University (Virtual)

11:30 am  12:00 pm ESTCoffee Break11th Floor Collaborative Space

12:00  12:30 pm ESTProject 3: Perspectives on Permutations11th Floor Lecture Hall
 Speakers
 Carolina Benedetti V., Universidad de los Andes
 Emily Gunawan, University of Oklahoma
 Jianping Pan, North Carolina State University
 Heather Russell, University of Richmond
 Bridget Tenner, DePaul University
 Session Chairs
 Samantha Dahlberg, Aurora University
 Susanna Fishel, Arizona State University (Virtual)

12:30  1:45 pm ESTLunch/Free Time

1:45  2:15 pm ESTProject 1: Investigating Convex Union Representable Complexes11th Floor Lecture Hall
 Speakers
 Nursel Erey, Gebze Technical University
 Susanna Fishel, Arizona State University
 Isabella Novik, University of Washington
 Casey Pinckney, University of Maine
 Session Chairs
 Pamela E. Harris, Williams College (Virtual)
 Caroline Klivans, Brown University

2:30  3:00 pm ESTProject 2: Rowstrict Dual Immaculate Functions11th Floor Lecture Hall
 Speakers
 Elizabeth Niese, Marshall University
 Sheila Sundaram, Pierrepont School
 Stephanie van Willigenburg, University of British Columbia
 Julianne Vega, Kennesaw State University
 Shiyun Wang, University of Southern California
 Session Chairs
 Pamela E. Harris, Williams College (Virtual)
 Caroline Klivans, Brown University

3:00  3:30 pm ESTCoffee Break11th Floor Collaborative Space

3:30  4:00 pm ESTProject 10: The A, C, Shifted BerensteinKirillov Groups and Cacti11th Floor Lecture Hall
 Speakers
 Olga Azenhas, University of Coimbra
 Mojdeh Tarighat, University of Virginia
 Jacinta Torres, Institute of Mathematics of the Jagiellonian University
 Session Chairs
 Pamela E. Harris, Williams College (Virtual)
 Caroline Klivans, Brown University

4:00  4:15 pm ESTGroup Photo (Immediately After Talk)11th Floor Lecture Hall

4:15  5:30 pm ESTWelcome ReceptionReception  11th Floor Collaborative Space

5:30  6:15 pm ESTRecommended Group Meeting TimeGroup Work
Friday, February 11, 2022

10:00  10:30 am ESTProject 5: Combinatorics of Convex Polytopes11th Floor Lecture Hall
 Speakers
 Margaret Bayer, University of Kansas
 Sandra Kingan, Brooklyn College, CUNY
 Sara Solhjem, Minnesota State University Moorhead
 Hailun Zheng, University of Copenhagen
 Session Chairs
 Nadia Lafreniere, Dartmouth College
 Rosa Orellana, Dartmouth College (Virtual)

10:40  11:10 am ESTProject 8: Higher Dimensional ChipFiring11th Floor Lecture Hall
 Speakers
 Sarah Brauner, University of Minnesota
 Galen DorpalenBarry, University of Minnesota
 Selvi Kara, University of Utah
 Caroline Klivans, Brown University
 Lisa Schneider, Salisbury University
 Session Chairs
 Nadia Lafreniere, Dartmouth College
 Rosa Orellana, Dartmouth College (Virtual)

11:15  11:45 am ESTCoffee Break11th Floor Collaborative Space

11:45 am  12:15 pm ESTProject 9: McKay Matrices for Monoids11th Floor Lecture Hall
 Speakers
 Georgia Benkart, University of WisconsinMadison
 Elizabeth Manosalva, Universidad de Talca
 Rosa Orellana, Dartmouth College
 Shraddha Srivastava, Uppsala University
 Session Chair
 Nadia Lafreniere, Dartmouth College

12:30  1:30 pm ESTLunch/Free Time

1:30  2:00 pm ESTProject 6: Enumerative Combinatorics with Filling of Polyominoes11th Floor Lecture Hall
 Speakers
 Zhanar Berikkyzy, Fairfield University
 Pamela E. Harris, Williams College
 Ying Anna Pun, University of Virginia
 Catherine Yan, Texas A&M University
 Chenchen Zhao, University of Southern California
 Session Chairs
 Elizabeth Niese, Marshall University
 Stephanie van Willigenburg, University of British Columbia (Virtual)

2:10  2:40 pm ESTProject 11: Combinatorial dimension formulas for algebraic splines11th Floor Lecture Hall
 Speakers
 Shaheen Nazir, Lahore University of Management Sciences
 Anne Schilling, UC Davis
 Julianna Tymoczko, Smith College
 Session Chairs
 Elizabeth Niese, Marshall University
 Stephanie van Willigenburg, University of British Columbia (Virtual)

2:50  3:00 pm ESTOrganizer's Closing RemarksClosing Remarks  11th Floor Lecture Hall
 Susanna Fishel, Arizona State University
 Pamela E. Harris, Williams College
 Rosa Orellana, Dartmouth College
 Stephanie van Willigenburg, University of British Columbia

3:00  3:30 pm ESTCoffee Break11th Floor Collaborative Space

3:30  5:00 pm ESTRecommended Group Meeting TimeGroup Work
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Project Descriptions
Project 1: Investigating Convex Union Representable Complexes, Isabella Novik, University of Washington
An abstract simplicial complex is called drepresentable if it is the nerve of a family of convex sets in the ddimensional Euclidean space. The research on drepresentable complexes has a rich and fascinating history, starting with Helly’s theorem, that to a large degree shaped the modern discrete geometry.
A simplicial complex is dconvex union representable (or dCUR, for short) if it arises as the nerve of a finite collection of convex open sets in R^d whose union is also convex. This is a fascinating and very natural subclass of representable complexes. In addition, the motivation for investigating such complexes comes from the theory of convex neural codes. While the class of drepresentable complexes is well and longstudied, the notion of dCUR complexes was introduced only a couple of years ago. Consequently, our present knowledge about this class of complexes is almost null. For instance, while we know that CUR complexes are collapsible and so are their Alexander duals, we do not know the answers to such questions as: is the barycentric subdivision of a CUR complex also CUR? Is every shellable simplicial ball CUR? We are also very short on techniques allowing to construct many such complexes.
The goal of the project will be to further explore the class of CUR complexes and probe some of the open problems.
Recommended background:
 Some knowledge of discrete geometry and polytopes
References
Aaron Chen, Florian Frick, and Anne Shiu. Neural codes, decidability, and a new local obstruction to convexity, SIAM J. Appl. Algebra Geom. 3 (1), 44−66 (2019)
Amzi Jeffs and Isabella Novik. Convex union representability and convex codes, Int. Math. Res. Not. IMRN, 2021, no. 9, 71327158
Martin Tancer. Intersection patterns of convex sets via simplicial complexes: a survey. In Thirty essays on geometric graph theory, pages 521–540. Springer, New York, 2013.
Project 2: Rowstrict Dual Immaculate Functions, Elizabeth Niese, Marshall University
There are a number of Schurlike bases for the space of quasisymmetric functions with combinatorial definitions reminiscent of the semistandard tableaux definition of symmetric Schur functions. One such basis, the dual immaculate functions [2], can be defined as the generating function of a set of composition tableaux of shape α where the first column of α is filled with positive integers so that it is increasing and each row is weakly increasing. This basis expands positively in the Young quasisymmetric Schur basis [1]. Just as the symmetric Schur functions can be defined as the generating function of rowstrict tableaux, with weakly increasing columns, there is a rowstrict quasisymmetric Schur basis [3]. In this project we will define a rowstrict version of the dual immaculate functions and then explore their combinatorics and algebra. Some directions for this research include finding an expansion in terms of the rowstrict quasisymmetric Schur functions, identifying Pieri rules, and finding 0Hecke modules with the rowstrict dual immaculate functions as their quasisymmetric characteristic.
References
[1] Edward Allen, Joshua Hallam, and Sarah Mason, “Dual immaculate quasisymmetric functions expand positively into Young quasisymmetric Schur functions,” J. Combin. Theory Ser. A 157 (2018), 70–108.
[2] Chris Berg, Nantel Bergeron, Franco Saliola, Luis Serrano, and Mike Zabrocki, A lift of the Schur and HallLittlewood bases to noncommutative symmetric functions, Canad. J. Math. 66 (2014), no. 3, 525–565.
[3] Sarah Mason and Jeffrey Remmel, Rowstrict quasisymmetric Schur functions, Ann. Comb. 18 (2014), no. 1, 127–148.
Project 3: Perspectives on Permutations, Bridget Tenner, DePaul University
Combinatorialists encounter permutations in many settings, two of which will be the foundation of this group's work. First, the symmetric group is the finite Coxeter group of type A, and this consists of all permutations of a given size. As a Coxeter group, this can be endowed with a natural poset structure (the Bruhat order), and many interesting mathematical questions  both answered and unanswered  can be posed about this structure. Second, one can look at patterns occurring within permutations, examining both the characterizing and the enumerative properties that result from such occurrences. A permutation pattern is typically phrased as a binary property: containment or avoidance. However, recent evidence has shown that a higherorder approach can provide an important and influential framework for understanding various phenomena, with the question being a count of how many times a permutation contains a pattern, or in what ways that containment occurs. Connections between the Coxeterperspective and the patternperspective have already been established, and this project aims to extend that relationship. Objects we will focus on may include ideals in the Bruhat order, commutation classes of reduced decompositions, and the RSK correspondence.
Project 4: Alternating Sign Matrices and Plane Partitions, Ilse Fischer, Universität Wien
In this project, we will focus on the mysterious relation between alternating sign matrices and plane partitions. Alternating sign matrices had been introduced in the 1980's and it was soon discovered (conjecturally) that they are counted by the same simple product formula as two classes of plane partitions, namely descending plane partitions and totally symmetric selfcomplementary plane partitions. Very recently, a fourth class of objects (alternating sign triangles) has been added to the list of objects that are counted by this product formula. There have been many efforts to construct bijective proofs, but so far there only exists a very complicated (recent) construction that proves bijectively an identity that implies the equinumerosity of alternating sign matrices and descending plane partitions.
We have two concrete projects in mind. Alternating sign matrices include permutation (matrices) as a special case, and for this special case bijections have been constructed in the work of Ayyer, of Fulmek and of Striker. The first possibility is to extend this to the next level of complication, by introducing a small number of 1's. Recent work of Höngesberg [3] actually does include 1's for alternating sign triangles, so that this work will be a good starting point. The other project is around unpublished work (at the moment) concerning n+3 pairs of equivalent statistics on two new types of objects that extend alternating sign matrices and descending plane partitions, respectively. That's exciting because for alternating sign matrices and descending plane partitions only four such statistics are currently known.
References
[1] David Bressoud, Proofs and confirmations. The story of the alternating sign matrix conjecture. MAA Spectrum. Mathematical Association of America and Cambridge University Press, Washington, DC and Cambridge, 1999.
[2] Ilse Fischer and Matja{\v z} Konvalinka. The mysterious story of square ice, piles of cubes, and bijections. Proceedings of the National Academy of Sciences, 117(38):23460–23466, 2020.
[3] Hans H{\"o}ngesberg, Weightpreserving bijections between integer partitions and a class of alternating sign trapezoids, arXiv:2102.07555v1.
Project 5: Combinatorics of Convex Polytopes, Margaret Bayer, University of Kansas
The study of the combinatorics of convex polytopes has a long and interesting history, with connections to operations research, algebra and algebraic geometry. There are many open questions in the field; this project will focus on enumeration questions. The face vector counts the number of faces of each dimension; the flag vector counts the numbers of chains of faces (ordered by inclusion) of specified dimensions. Both are characterized for 3dimensional polytopes, but there is not even a conjectured characterization for general 4dimensional polytopes. The affine spans of the face vectors and flag vectors of polytopes of dimension d are known. Many inequalities are known. Characterization of both sets of vectors are known for simplicial polytopes, and some results are known for other classes of polytopes. Methods come from combinatorial analysis, but also by applying results in algebra, algebraic geometry, and algebraic topology to polytopes. Depending on the participants' interests and expertise, the project will focus either on a particular class of polytopes or on particular methods.
References
Ron Adin, Daniel Kalmanovich, Eran Nevo, On the cone of fvectors of cubical polytopes, Sem. Lothar. Combin. 80B (2018), Art. 85, arXiv:1801.00163v2
Louis J. Billera, "Even more intriguing, if rather less plausible..." Face numbers of convex polytopes, in: The Mathematical Legacy of Richard P. Stanley, 6581, AMS, 2016.
Eran Nevo, Complexity yardsticks for fvectors of polytopes and spheres, Discrete Comput. Geom. 64 (2020), no. 2, 347354, arXiv:1908.09628
Isabella Novik, Hailun Zheng, The stresses on centrally symmetric complexes and the lower bound theorems, arXiv:2008.12503
Lei Xue, A proof of Grunbaum's lower bound conjecture for general polytopes, arXiv:2004.08429
Project 6: Enumerative Combinatorics with Filling of Polyominoes, Catherine Yan, Texas A&M University
A polyomino is a finite subset of Z2 , where each element is represented by a square cell. The combinatorial model is obtained by taking a connected polyomino and assigning a nonnegative integer to each cell. By considering different shapes, convexity, symmetry, and content of the filling, we get a unified model that contains a hierarchy of combinatorial structures. Explicitly, we would consider the following families of polyominoes, including
 square or rectangular shaped polyominoes whose fillings correspond to permutations and words;
 Ferrers diagrams which are polyominoes given by an integer partition. Fillings of Ferrers diagrams correspond to matchings, set partitions, and general graphs;
 stack polyominoes which are convex, intersectionfree, and whose rows are arranged monotonically;
 moon polyominoes which are convex and intersectionfree;
 more general shape of polyominoes with weaker convex properties.
For fillings, we consider two basic cases: the one using 0 and 1, and the one using arbitrary natural numbers. These fillings, together with the aforementioned polyominoes, allow us to use combinatorial operations and transformations to investigate special characteristics of each family. It is particularly convenient to study patterns and combinatorial statistics.
Another approach to fillings of polyominoes is to apply the tools developed in the study of symmetric functions and standard Young tableaux, for example, the RobinsonSchenstedlike insertion/deletion processes, growth diagram, and the Knuth equivalence. Such techniques are particularly powerful in dealing with the maximal monotone substructures, such as increasing and decreasing subsequences in integer sequences, and the maximal crossings and nestings in various diagrams.
The objective of this project is to understand the interplay between the combinatorial transformations and the tableaux operations, and to characterize various combinatorial statistics in different families of fillings of polyominoes.
Background needed: Basic techniques in enumerative combinatorics; be familiar with permutation statis tics. A basic knowledge of symmetric functions, STYs, and the RSK algorithm would be helfpul, (as in Stanley’s EC2.)
Project 7: Dynamical Algebraic Combinatorics, Jessica Striker, North Dakota State University
Do you have a favorite set of combinatorial objects S with a natural bijective action f:S>S? Then come investigate its dynamics with us!
We'll begin the project by discussing everyone's favorite objects and actions in the context of the questions listed below. We will then do some computational exploration, as described below each question. Based on the computations, we will concentrate on studying the most promising actions.
Questions to ask about your favorite objects and actions:
 Does your action have a predictable order?
 If so, see if it exhibits the cyclic sieving phenomenon [5] (can predict orbit structure by evaluating a generating function at a certain root of unity).
 If not, see if it exhibits resonance [2] (projects to another object/action pair with a predictable order).
 Does your object have important statistics?
 If so, take a look at how those statistics interact with the action. It may be that they exhibit homomesy [4] (same average value on all orbits).
 Is your set of objects equivalent to a set of subsets?
 If so, study its generalized toggle group actions (some examples of this are given in [9, Section 3]).
 Is your action equivalent to the composition of small local actions?
 If so, characterize it as a toggle group action [10].
This project will involve computational aspects, so programming experience would be helpful. But it will also be a great way to develop your programming skills in a supportive environment, so prior experience is not necessary. References [8] and [6] are survey articles that give an overview of the area.
References
[1] J. Bernstein, J. Striker, and C. Vorland. Pstrict promotion and Bbounded rowmotion, with applications to tableaux of many flavors. https://arxiv.org/abs/2012.12219 (submitted).
[2] K. Dilks, O. Pechenik, and J. Striker. Resonance in orbits of plane partitions and increasing tableaux. J. Combin. Theory Ser. A, 148 (2017), 244274.
[3] K. Dilks, J. Striker, and C. Vorland. Rowmotion and increasing labeling promotion. J. Combin. Theory Ser. A, 164 (2019), 72108.
[4] J. Propp and T. Roby. Homomesy in products of two chains. Electron. J. Combin., 22 (2015), no. 3.
[5] V. Reiner, D. Stanton, and D. White. The cyclic sieving phenomenon. J. Combin. Theory Ser. A, 108 (2004), no. 1, 1750.
[6] T. Roby. Dynamical Algebraic Combinatorics and the Homomesy Phenomenon. A. Beveridge, J. Griggs, L. Hogben, G. Musiker, P. Tetali, eds., Recent Trends in Combinatorics (IMA Volume in Mathematics and its Applications), Springer, 2015.
[7] J. Striker and N. Williams. Promotion and rowmotion. Eur. J. Combin., 33 (2012), no.8, 19191942.
[8] J. Striker. Dynamical algebraic combinatorics: promotion, rowmotion, and resonance. Notices of the American Mathematical Society, 64 (2017), no. 6, 543549.
[9] J. Striker. Rowmotion and generalized toggle groups. Discret. Math. Theor. Comput. Sci., 20 (2018), no. 1.
[10] J. Striker. The toggle group, homomesy, and the RazumovStroganov correspondence. Electron. J. Combin., 22 (2015), no. 2.
Project 8: Higher Dimensional ChipFiring, Caroline Klivans, Brown University
Chipfiring processes are combinatorial dynamical systems. Traditionally the dynamics are envisioned on a graph or network. A commodity (chips, sand, dollars) is exchanged between sites of a network according to simple local rules. Although governed by local rules, chipfiring systems result in compelling global behavior.
More recent work has sought to broaden the domain of chipfiring to other settings such as root systems, Mmatrices and matroids. This project will focus on chipfiring in higherdimensions. Higher dimensional chipfiring can be thought of as a flowrerouting process on cellular complexes. A commodity (now thought of as a flow) is diffused along the faces of a cell complex. The diffusion is dictated by the structure of the complex or equivalently by a combinatorial Laplacian. We will investigate questions about the behavior of such systems, related for example to stabilization, confluence and expansion.
Preferred background: Comfort with basic programming in a language such as matlab or python will be helpful but is not necessary. Someone(s) with skills in visualization would be excellent.
References
A. Duval, C. Klivans, and J. Martin, Critical groups for simplicial complexes. Annals of Combinatorics vol. 17, Issue 1, 2013.
P. Felzenszwalb and C. Klivans, Flowfiring processes. Journal of Combinatorial Theory A, vol. 177, 2021.
C. Klivans, The Mathematics of Chipfiring, Publisher: CRC Press. 2018.
Project 9: McKay Matrices for Super Objects and Connections with Characters, Georgia Benkart, University of WisconsinMadison
The McKay matrix records the result of tensoring the finitedimensional simple modules with a fixed finitedimensional module V. In the case of a finite group G and its complex modules, the columns of the character table of G give a complete set of right eigenvectors for the McKay matrix determined by any finitedimensional Gmodule V. The corresponding eigenvalues depend on V and are the values of the character of V on a set of conjugacy class representatives of G. This project involves investigating the analogous situation for supergroups, for small Lie superalgebras such as gl(1,1) and osp(1,2), and for their quantum analogues defined using roots of unity.
The centralizer algebra of transformations that commute with the action of the (super)group or (super)algebra on tensor powers of the module V often have significant connections with diagram algebras and braid groups. Another goal is to investigate the centralizer algebras that arise from studying these super objects and their representations.
Project 10: The A, C, Shifted BerensteinKirillov Groups and Cacti, Olga Azenhas, University of Coimbra
This project studies a symplectic version of the BerensteinKirillov group, BKC, and its relationship with the combinatorial internal action of the cactus group Jsp2n, constructed by Halacheva [HKRW20] using the Dynkin diagram I = [n] of the root system in type Cn, on a normal crystal of type Cn via partial Sch ̈utzenbergerLusztig involutions.
We focus on the type Cn crystals of KashiwaraNakashima tableaux [KN94, L07]. They are compatible with a plactic monoid, bumping algorithm and symplectic jeu de taquin [L07]. Considering KN = KN(λ, n) to be the crystal of KashiwaraNakashima tableaux of straight shape λ on the alphabet [±n], a full evacuation algorithm [S21], whose construction depends on the long element of the Weyl group W = Bn (the hyperoctahedral group), also exists and agrees with the Lusztig involution on the set KN. For 1 ≤ q < n, the crystal KNq, the restriction of the crystal KN to the connected subdiagram [1, q] of the Dynkin diagram I, is of type Aq with Weyl group Sq+1. Partial evacuation algorithms agreeing with the Lusztig involution on the set KNq are not known. The action of the cactus group Jsp2n on KN makes manifest the action of the Weyl group Bn on that crystal but symplectic BenderKnuth involutions for KashiwaraNakashima tableaux are not known. However, the action of certain elements of the cactus group Jn = Jgln on a straight shape semistandard Young tableau (or GillespieLevinsonPurbhoo shifted tableau) crystal is (shifted) Bender Knuth like [BK95, R21]. We hope that a similar behaviour of the action of certain elements in the cactus Jsp2n on KN gives an indication on what a symplectic Bender Knuth involution for the KashiwaraNakashima tableaux should be.
The BerensteinKirillov group or GelfandTsetlin group, BK, studied by Berenstein and Kirillov [BK95], is the free group generated by the BenderKnuth involutions, modulo the relations they satisfy on semistandard Young tableaux of any shape. Fixing the subgroup BKn, it has an alternative set of generators whose action on the straight shaped semistandard Young tableau crystal SSYT(λ, n) agrees with the partial Sch ̈utzenbergerLusztig involutions, and, therefore, coincides with the internal action of the cactus group Jn on SSYT(λ, n), studied by Halacheva [H20]. Chmutov, Glick and Pylyavskii [CGP20] also related the BerensteinKirillov group and the cactus group Jn using semistandard growth diagrams, and determined implications between subsets of relations in the two groups, which yields a presentation for cactus groups Jn in terms of BenderKnuth generators. In the vein of Halacheva and ChmutovGlickPylyavskii, a shifted version of the BerensteinKirillov group has been studied in [R21] considering shifted BenderKnuth involutions and an action of the cactus group Jn on a GillespieLevinsonPurbhoo shifted tableau crystal [GLP17]. A similar presentation of the cactus group Jsp2n in terms of symplectic BenderKnuth generators also makes sense to be explored.
Suggested background: Combinatorics of crystal graphs and tableau combinatorics along the lines of D. Bump and A. Schilling’s book ”Crystal Bases. Representations and Combinatorics”.
References
[BK95] A. D. Berenstein and A. N. Kirillov. Groups generated by involutions, Gelfand–Tsetlin patterns, and combinatorics of Young tableaux, Algebra i Analiz, 7, 1995, 1, 92–152.
[CGP20] M. Chmutov and M. Glick and P. Pylyavskyy. The BerensteinKirillov group and cactus groups, J. Combinatorial Algebra, 2020,4,2, 111–140, arXiv:1609.02046v2.
[GLP17] M. Gillespie, J. Levinson. Kevin Purbhoo. A crystallike structure on shifted tableaux, J. Algebraic Combinatorics, 3, 2020, 693–725, arXiv:1706.09969.
[H20] I. Halacheva. Skew Howe duality for crystals and the cactus group, arxiv 2001.02262v1.
[HKRW20] I. Halacheva and J. Kamnitzer and L. Rybnikov and A. Weeks. Crystals and monodromy of Bethe vectors, 169, Duke Math. Journal, 12, 2337 – 2419, 2020.
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Project 11: Combinatorial dimension formulas for algebraic splines, Julianna Tymoczko, Smith College
Splines are a fundamental tool in applied mathematics and analysis, classically described as piecewise polynomials on a combinatorial decomposition of a geometric object (a triangulation of a region in the plane, say, or a convex polytope) that agree up to a specified differentiability on faces of codimension one. Generalized splines extend this idea algebraically and combinatorially: instead of certain classes of geometric objects, we start with an arbitrary combinatorial graph; instead of labeling faces with polynomials, we label vertices with elements of an arbitrary ring; and instead of applying degree and differentiability constraints, we require that the difference between ring elements associated to adjacent vertices be a multiple of a fixed ring element labeling the edge. Billera showed that these two characterizations coincide in most cases of realworld interest.
In this project, we’ll try to make headway on the socalled upper bound conjecture from analysis, which asks for the dimension of the collection of splines on certain triangulations in the plane. We’ll think about generalized splines
 on planar trivalent graphs
 whose edges are labeled by twovariable polynomials of the form (ax+by)^2
 and whose vertices are labeled by polynomials of degree at most two
Our goal is to conjecture and/or prove formulas for the dimension of the space of splines in as special or general context as we can, whether in terms of combinatorial features of the graph, or for certain families of trivalent graphs, or by developing algorithms that crawl through the graphs in particular ways or using other strategies that we come up with.
References
Gilbert, Tymoczko, Viel. Generalized splines on arbitrary graphs. Pacific Journal of Math, Vol. 281 (2016), No. 2, 333364.
Anders, Crans, FosterGreenwood, Mellor, Tymoczko. Graphs admitting only constant splines. Pacific Journal of Math, Vol. 304 (2020), No. 2, 385400.