Schubert Varieties (2017)

Venue: The Institute of Mathematical Sciences,  Chennai, Tamil Nadu
Date:  23rd, Oct 2017 to 4th, Nov 2017

Convener(s)
Name Sudhir R. Ghorpade K. N. Raghavan
Mailing Address Department of Mathematics
Indian Institute of Technology Bombay
Powai, Mumbai 400 076
The Institute of Mathematical Sciences
C. I. T. Campus, Taramani
Chennai 600 113

 


 

Speakers and Syllabus 

The intention was to cover as much of Laurent Manivel’s monograph on the subject. There are three chapters in that monograph and correspondigly there were three courses in the workshop.

Symmetric functions and the representation theory of the symmetric group (the first chapter) was covered by AMRITANSHU PRASAD and SANKARAN VISWANATH. The topic of Schubert polynomials (the second chapter) was covered by VIJAY RAVIKUMAR and K. N. RAGHAVAN. The geometry of Schubert varieties (the third chapter) was covered by SUDHIR GHORPADE and EVGENY SMIRNOV. Three lectures on cohomology and characteristic classes in general were given by PARAMESWARAN SANKARAN.

Listed below in separate subsections are the synopses of the courses in the words of the respective speakers.

Amritanshu Prasad (8 lectures of 90 minutes each).

  • Lecture 1
    • Symmetric polynomials
    • Elementary and complete symmetric polynomials
    • Transition from monomial to elementary and complete polynomials
  • Lecture 2:
    • Alternating polynomials
    • Bialternant form of Schur polynomials
    • Basis of Schur polynomials
  • Lecture 3:
    • Abaci and alternants
    • Pieri’s formula
    • Transition from Schur to elementary and symmetric polynomials
    • Semistandard tableaux, Kostka numbers
  • Lecture 4:
    • Triangularity of Kostka numbers.
    • Schensted insertion
    • Insertion tableau of a word
  • Lecture 5:
    • The plactic monoid
    • Greene’s theorems
    • Section theorem
  • Lecture 6:
    • Plactic Pieri rules
    • Kostka’s definition of Schur functions
  • Lecture 7:
    • Lindstrom-Gessel-Viennot Lemma
    • Jacobi-Trudi identities
    • Giambelli’s formula
  • Lecture 8:
    • RSK correspondence
    • The Littlewood-Richardson rule

Sankaran Viswanath (4 lectures of 90 minutes each). Review of the theory of finite dimensional complex representations of finite groups:

  • Symmetric polynomials as characters of polynomial representations of GL(V ).
  • The Frobenius characteristic map and the relation between the characters of the symmetricgroup and symmetric functions.
  • Construction of the irreducible representations of the symmetric group; Specht modules.
  • The restriction problem for irreducible representations of symmetric groups.

Vijay Ravikumar (4 lectures of 90 minutes each).

  • Lecture 1: Rothe Diagrams, Lehmer codes, essential boxes, rank functions, length of permutation
    • generators and relations for Sn, reduced decompositions of permutations
    • a canonical reduced decomposition (’Lascoux’s magic trick”)
    • the graph of reduced words for a given permutation
    • the weak and strong Bruhat orders on Sn
    • statement of subword property
  • Lecture 2: More on the Bruhat order
    • writing the inversion set based on a given decomposition of a permutation
    • the strong exchange property, statement and proof
    • Bruhat order in terms of key tableaux
    • Bruhat order in terms of associated rank functions
    • definition of Grassmannian permutations
  • Lecture 3: Wiring diagrams, Divided difference operators
    • conclusion of proof of subword property
    • wiring diagrams for permutations
    • proof that the graph of reduced words is connected
    • divided difference operators
    • definition and uniqueness of Schubert polynomials
  • Lecture 4: Computing Schubert polynomials
    • existence of Schubert polynomials (i.e. stability under inclusion of Sn into Sn+1)
    • computing Schubert polynomials using divided difference operators
    • Fomin-Kirillov configurations (i.e. wiring diagrams written a special way)
    • computing a Schubert polynomial by resolving crossings in a particular wiring diagram
    • Stanley’s conjecture for the Schubert polynomial in terms of compatible sequences

K. N. Raghavan (5 lectures of 90 minutes each). The topics covered were:

  • Basic facts about Schubert polynomials: for instance, they form an integral basis for the ring Z[x1, x2, . . .] over the integers in countably infinitely many variables.
  • Monk’s rule for the multiplication of a Schubert polynomial with xi
  • The nil-Hecke algebra and the Yang-Baxter equation: applications to the computation of double Schubert polynomials: justification for the Fomin-Kirillov procedure and in turn the proof of Stanley’s conjecture.
  • Facts about the co-invariant ring for the action of the symmetric group on n letters on the polynomial ring in n variables: for instance, that it is the regular representation (as a module for the group).
  • Cauchy’s formula for double Schubert polynomials.

Sudhir Ghorpade (6 lectures of 90 minutes each).

  • Lecture 1: Introduction to Grassmannians, Plucker embedding, Grassmannian as a homogeneous space (transitive group action of the General linear group), Plucker relations
  • Lecture 2: Sufficiency of Plucker relations, Ideal of the Grassmannian. Schubert cells and Schubert varieties in Grassmannians, Cellular decomposition of the Grassmannian,
  • Lecture 3: Cellular decomposition of Schubert varieties. Inclusions of Schubert varietiesin Grassmannians and the Bruhat order, Poincare polynomial of the Grassmannian, Connection with the number of Fq-rational points of the Grassmannian. Schubert varieties as linear sections of Grassmannians.
  • Lecture 4: Standard monomials, Hodge basis theorem for the homogeneous coordinate ring of Grassmannians and more generally, for the homogeneous coordinate ring of Schubert varieties in Grassmannians. Applications to the determination of the vanishing ideal of Grassmannians. Hodge postulation formula for Schubert varieties in Grassmannians.
  • Lecture 5: Cohomology ring of the Grassmannian (with integer coefficients), Schubert cycles, Cup products, Intersections of Schubert varieties. Cup products of Schubert classes of complementary codimension.
  • Lecture 6: Pieri’s formula for Schubert classes. Applications: Relation of Schur functions and Schubert classes (in Grassmannians), Giambelli’s formula, Product rule for Schubert classes.

 Evgeny Smirnov (6 lectures of 90 minutes each). Singularities of Schubert varieties:

  • Lecture 1: Schubert cells and Schubert varieties in Grassmannians. Schubert varieties and enumerative geometry. Example: the four lines problem. Bott-Samelson resolutions of singularities of Schubert varieties.• Lecture 2: Bott-Samelson resolutions of singularities of Schubert varieties. Smoothness criterion for Schubert varieties. Poincare polynomial for a Grassmannian, q-binomial coefficients. Small resolutions of singularities.
  • Lecture 3: Singular loci of Schubert varieties: Tangent space to a Schubert variety
  • Lecture 4: Singular loci of Schubert varieties (continued): Singularity criterion, codimension of the singular locus. Degree of a Schubert variety.
  • Lecture 5: Flag varieties. Schubert varieties, Schubert classes, Bruhat order. The Schubert classes form a self-dual basis.
  • Lecture 6: Monk’s rule. Recall on Chern classes. Borel’s presentation of the cohomology ring of a full flag variety.

Parameswaran Sankaran (3 lectures of 90 minutes each).

  • Lecture 1: Singular homology and cohomology. Cellular (co)homology of a (finite) CW complex. Relation to de Rham cohomology of smooth manifolds. Orientability of a manifold. Ring structure of cohomology and Poincare duality. Cohomology class dual to a  ́submanifold. Cohomology ring of a complex projective space.
  • Lecture 2: Cohomological implications of Kahler structure on a compact complex manifold. Holomorphic line bundles and their Chern classes.
  • Lecture 3: The cohomology ring of the infinite complex Grassmannian as the ring of symmetric polynomials in several variables. Cohomology ring of complex Grassmann manifolds.

Resource persons and subjects

  1. Amritanshu Prasad (AP) Symmetric functions
  2. Sankaran Viswanath (SV): symmetric functions (continued)
  3. Vijay Ravikumar (VR): Schubert polynomials
  4. K. N. Raghavan (KNR): Schubert polynomials (continued)
  5. Sudhir Ghorpade (SG): geometry of Schubert varieties
  6. Evgeny Smirnov (ES): geometry of Schubert varieties (continued)
  7. Parameswaran Sankaran (PS): pre-requisite topological background

 

Time Table

    Lecture 1 Lecture 2 Lecture 3 Discussion Hr.
Day Date 0930–1100 1130–1300 1400 to 1530 1600–1700
1 23rd Oct I III V I, III, and V
2 24th Oct I III V I, III, and V
3 25th Oct I III V I, III, and V
4 26th Oct I III VII I, III, and V
5 27th Oct I III VII I, III, and V
6 28th Oct I III V I, III, and V
7 30th Oct I VII VI I, IV, and VI
8 31st Oct I IV VI I, IV, and VI
9 1st Nov II IV VI II, IV, and VI
10 2nd Nov II VI IV II, IV, and VI
11 3rd Nov II IV VI II, IV, and VI
12 4th Nov II VI IV II, IV, and VI

 

Actual Participants 

 

Sr. SID Full Name Gender Affiliation Position in College/ University University / Institute M.Sc./M.A. Year of Passing M.Sc./ M.A Ph.D. Deg.
Date
Selected (outstation) applicants with accomodation
1 13773 Mr. Arijit Mukherjee Male HYDERABAD CENTRAL UNIVERSITY PhD BALLYGUNGE SCIENCE COLLEGE,UNIVERSITY OF CALCUTTA 2014  
2 13778 Mr. Nitin Shridhar Darkunde Male School of Mathematical Sciences, SRTM University, Nanded. Assistant Professor Department of Mathematics, Savitribai Phule Pune University, Pune. 2008  
3 14139 Dr. Venkatesh R Male Indian Institute of Science Assistant Professor   2009 09/27/2013
4 14260 Mr Narasimha Chary Male Institut Fourier,
University of Grenoble Alpes.
Post Doctoral researcher University of Hyderabad 2009 05/01/2016
5 14781 Ms. Arpita Nayek Female IIT KANPUR PhD IIT MADRAS. M.Sc 2014  
6 14797 Mr. Basudev Pattanayak Male IISER, Pune PhD IIT BOMBAY 2016  
7 14857 Mr. Amith K Shastri Male TIFR PhD University of Madras 2014  
8 15342 Mr. Pritam Majumder Male TIFR PhD IIT Kanpur 2014  
9 15385 Mr. Ajay Kumar Male HNB Garhwal University (A centar university) Srinagar (Garhwal) Uttarakhand University HNB Garhwal University (A centar university) Srinagar (Garhwal) Uttarakhand 2014  
10 15426 Dr. Ambily A A Female Cochin University of Science and Technology Assistant Professor Cochin University of Science and Technology, Kerala 2007 07/18/2014
11 15595 Ms. Rosna Paul Female Cochin University of Science and Technology MPhil Cochin University of Science and Technology 2017  
12 15635 Ms Saudamini Nayak Female HRI Post Doctoral Fellow Sambalpur University 2008 09/09/2016
13 15697 Dr. Prasant Singh Male IIT Bombay PhD MSc 2010  28/6/2017
14 15747 Mr.Avijit Panja Male IIT Bombay Ph.D student Ramakrishna Mission Vidyamandira, Belur 2010  
15 15888 Mr.Biswajit Ransingh  Male  Harish Chandra Research Institute Post-doctoral Fellow  NIT Rourkela    13 May 2014
16 15911 Dr. Shameek Paul  Male Homi Bhabha Centre for Science Education Visiting Fellow University of Mumbai 2005 7 Jan 2012
Selected (local) applicants without accomodation
1 13756 Mr Sarjick Bakshi Male Chennai Mathematical Institute PhD Chennai Mathematical Institute 2014  
2 14293 Dr. B Ravinder Male Chennai Mathematical Institute INSPIRE Faculty University of Hyderabad 2009 04/29/2016
3 14512 Mr Pinakinath Saha Male Chennai Mathematical Institute PhD Visva-Bharati 2015  
4 14910 Ms Shraddha Srivastava Female Chennai Mathematical Institute PhD M.Sc. 2011  
5 15322 Mr. Krishanu Roy Male Institute of Mathematical Sciences PHD Institute of Mathematical Sciences 2015  
6 15380 Mr. Subham Bhakta Male Chennai Mathematical Institute MSc Student Chennai Mathematical Institute Appeared  
7 15470 Mr. Aditya N K Subramaniam Male Chennai Mathematical Institute PhD Savitribai Phule Pune University 2015  
8 15543 Mr. Amit Kamar Singh Male IIT Madras PhD M.Sc. 2011  
9 15760 Mr. Digjoy Paul Male IMSC PhD IIT MADRAS 2014  
10 15787 Mr Mrigendra Singh Kushwaha Male Institute of Mathematical Sciences PhD M.Sc. 2012  
11 - Mr. Arun Kumar Male IMSc - - - -
12   Jayakumar R Male IMSc JRF Ramanujan Institute for Advanced Study in Mathematics, University of Madras 2016 joined 2016 August.
13   Narayanan P. A. Male IMSc N/A   2012  
14   Sruthymurali Female IMSc Research Fellow   2014 pursuing

 


Link to site:
https://www.imsc.res.in/~knr/past/2017_sch_atmw/index.html


How to reach

The Chennai neé Madras, and to IMSc, the Institute of Mathematical Sciences. Here is some information on getting to the Institute. Address and phone numbers are

THE INSTITUTE OF MATHEMATICAL SCIENCES
C I T CAMPUS, TARAMANI, CHENNAI 600 113
Telephone Numbers

Here are some maps to give you a rough idea of the layout of Madras and the area around the Institute.

The Institute of Mathematical Sciences is located in a campus-like area variously known as the ``CIT Campus'' or the ``CPT Campus''. Many people in Madras may not have heard of the Institute, but almost everyone would have heard of the M.G.R. Film City and Tidel Park which are further down in the same campus area. IMSc has two complexes. One houses the main Institute building, the library building and the new building. The other has the students' hostel and Guest House. These two complexes are across the road from each other.