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
 Speaker
 Poster Presenter
 Attendee
 Virtual Attendee

Olga Azenhas
University of Coimbra

Margaret Bayer
University of Kansas

Carolina Benedetti V.
Universidad de los Andes

Georgia Benkart
University of WisconsinMadison

Zhanar Berikkyzy
Fairfield University

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
Colorado State University / University of Maine

Ying Anna Pun
University of Virginia

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

Catherine 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

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

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
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

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

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
All event times are listed in ICERM local time in Providence, RI (Eastern Daylight Time / UTC4).
All event times are listed in .
ICERM local time in Providence, RI is Eastern Daylight Time (UTC4). Would you like to switch back to ICERM time or choose a different custom timezone?
Application Information
ICERM welcomes applications from faculty, postdocs, graduate students, industry scientists, and other researchers who wish to participate. Some funding may be available for travel and lodging. Graduate students who apply must have their advisor submit a statement of support in order to be considered.
Applications are not currently open. Please check back at a later date.
Your Visit to ICERM
 ICERM Facilities
 ICERM is located on the 10th & 11th floors of 121 South Main Street in Providence, Rhode Island. ICERM's business hours are 8:30am  5:00pm during this event. See our facilities page for more info about ICERM and Brown's available facilities.
 Traveling to ICERM
 ICERM is located at Brown University in Providence, Rhode Island. Providence's T.F. Green Airport (15 minutes south) and Boston's Logan Airport (1 hour north) are the closest airports. Providence is also on Amtrak's Northeast Corridor. Indepth directions and transportation information are available on our travel page.
 Lodging
 ICERM's special rate will soon be made available via this page for our preferred hotel, the Hampton Inn & Suites Providence Downtown. ICERM also regularly works with the Graduate Hotel and Hilton Garden Inn who both have discounted rates available. Contact housing@icerm.brown.edu before booking anything.
The only way ICERM participants should book a room is through the hotel reservation links located on this page or through links emailed to them from an ICERM email address (first_last@icerm.brown.edu). ICERM never works with any conference booking vendors and never collects credit card information.
 Childcare/Schools
 Those traveling with family who are interested in information about childcare and/or schools should contact housing@icerm.brown.edu.
 Technology Resources
 Wireless internet access ("BrownGuest") and wireless printing is available for all ICERM visitors. Eduroam is available for members of participating institutions. Thin clients in all offices and common areas provide open access to a web browser, SSH terminal, and printing capability. See our Technology Resources page for setup instructions and to learn about all available technology.
 Accessibility
 To request special services, accommodations, or assistance for this event, please contact accessibility@icerm.brown.edu as far in advance of the event as possible. Thank you.
 Discrimination and Harassment Policy
 ICERM is committed to creating a safe, professional, and welcoming environment that benefits from the diversity and experiences of all its participants. Both the Brown University "Code of Conduct", "Discrimination and Workplace Harassment Policy", and "Title IX Policy" apply to all ICERM participants and staff. Participants with concerns or requests for assistance on a discrimination or harassment issue should contact the ICERM Director or Assistant Director of Finance & Administration; they are the responsible employees at ICERM under this policy.
 Exploring Providence
 Providence's worldrenowned culinary scene provides ample options for lunch and dinner. Neighborhoods near campus, including College Hill Historic District, have many local attractions. Check out the map on our Explore Providence page to see what's near ICERM.
Visa Information
Contact visa@icerm.brown.edu for assistance.
 Eligible to be reimbursed
 B1 or Visa Waiver Business (WB)
 Ineligible to be reimbursed
 B2 or Visa Waiver Tourist (WT)
 Already in the US?

F1 and J1 not sponsored by ICERM: need to obtain a letter approving reimbursement from the International Office of your home institution PRIOR to travel.
H1B holders do not need letter of approval.
All other visas: alert ICERM staff immediately about your situation.
ICERM does not reimburse visa fees. This chart is to inform visitors whether the visa they enter the US on allows them to receive reimbursement for the items outlined in their invitation letter.
Financial Support
 ORCID iD
 As this program is funded by the National Science Foundation (NSF), ICERM is required to collect your ORCID iD if you are receiving funding to attend this program. Be sure to add your ORCID iD to your Cube profile as soon as possible to avoid delaying your reimbursement.
 Acceptable Costs

 1 roundtrip between your home institute and ICERM
 Flights on U.S. or E.U. airlines – economy class to either Providence airport (PVD) or Boston airport (BOS)
 Ground Transportation to and from airports and ICERM.
 Unacceptable Costs

 Flights on nonU.S. or nonE.U. airlines
 Flights on U.K. airlines
 Seats in economy plus, business class, or first class
 Change ticket fees of any kind
 Multiuse bus passes
 Meals or incidentals
 Advance Approval Required

 Personal car travel to ICERM from outside New England
 Multipledestination plane ticket; does not include layovers to reach ICERM
 Arriving or departing from ICERM more than a day before or day after the program
 Multiple trips to ICERM
 Rental car to/from ICERM
 Flights on a Swiss, Japanese, or Australian airlines
 Arriving or departing from airport other than PVD/BOS or home institution's local airport
 2 oneway plane tickets to create a roundtrip (often purchased from Expedia, Orbitz, etc.)
 Reimbursement Requests

Request Reimbursement with Cube
Refer to the back of your ID badge for more information. Checklists are available at the front desk and in the Reimbursement section of Cube.
 Reimbursement Tips

 Scanned original receipts are required for all expenses
 Airfare receipt must show full itinerary and payment
 ICERM does not offer per diem or meal reimbursement
 Allowable mileage is reimbursed at prevailing IRS Business Rate and trip documented via pdf of Google Maps result
 Keep all documentation until you receive your reimbursement!
 Reimbursement Timing

6  8 weeks after all documentation is sent to ICERM. All reimbursement requests are reviewed by numerous central offices at Brown who may request additional documentation.
 Reimbursement Deadline

Submissions must be received within 30 days of ICERM departure to avoid applicable taxes. Submissions after thirty days will incur applicable taxes. No submissions are accepted more than six months after the program end.
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.
[KN94] M. Kashiwara and T. Nakashima. Crystal graphs for representations of the qanalogue of classical Lie algebras. J. Algebra, 165, 2 (1994), pp. 295–345.
[L07] C. Lecouvey. Combinatorics of crystal graphs for the root systems of types An, Bn, Cn, Dn, G2, in Combinatorial Aspects of Integrable Systems, MSJ Memoirs vol 17, 11–41, 2007.
[R21] I. Rodrigues. A shifted BerensteinKirillov group and the cactus group, arXiv:2104.11799.
[S21] J. M. Santos. Symplectic keys and Demazure atoms in type C, to appear in Electron. J. Comb, 2021, arXiv:1910.14115v3.
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.