Things I should know about category O but I don't (Summer 2020)
 Organizers: Cailan Li, Henry Liu, Mrudul Thatte
 Time/date: Fridays 3:10pm  4:50pm Eastern Time (UTC−5:00)
 Location: online
This is an online learning seminar on category \(\mathcal{O}\). The plan is to roughly follow the book
and then branch out to other topics relating to category \(\mathcal{O}\) once we have gone over the fundamentals. A more detailed plan for the seminar is available here (PDF).
Please email Cailan at ccl at math dot columbia dot edu if you would like to join the seminar.
Rules for the seminar
 You must have an example/computation in \(\mathfrak{sl}_2\) or \(\mathfrak{sl}_3\) in your talk.
 You cannot give a slides talk unless your talk does not have an example/computation in \(\mathfrak{sl}_2\) or \(\mathfrak{sl}_3\).
 Turn your video on (for the most part) as a courtesy to the speaker.
 There are no dumb questions.
Please send your title and abstract to Cailan by Wednesday night, and your notes to Henry before your talk starts. If by some miracle you have your notes written and talk prepared by the end of Wednesday, you can include the notes in your email to Cailan.
Schedule
Fri Jul 17  Organizational meeting 
Fri Jul 24 
Dinushi Munasinghe Category O: Properties We will begin by defining the BGG Category \(\mathcal{O}\) and giving some basic properties. We will define highest weight modules, show that they are in \(\mathcal{O}\), and prove that they are indecomposable. We will then introduce the central characters, the dot action and the HarishChandra homomorphism to prove that \(\mathcal{O}\) is of finite length. Finally, we will introduce the subcategories \(\mathcal{O}_{\chi}\) and show that \(\mathcal{O}_{\chi_\lambda}\) is a block of \(\mathcal{O}\) for integral \(\lambda\). Notes: here (PDF) References: [H] 
Fri Jul 31 
Cailan Li Category O: Methods We first leisurely compute some Ext groups between our favorite objects in Category \(\mathcal{O}\). Afterwards, we will introduce a self duality functor on Category \(\mathcal{O}\) and compute some examples. Finally we will introduce standard filtrations on modules in Category \(\mathcal{O}\) and show that the multiplicity of a Verma module in a standard filtration of a module can be computed in terms of the duality functor introduced before. Time permmiting, we will introduce \(\rho\)dominant weights and show Category \(\mathcal{O}\) has enough projectives. Notes: here (PDF) References: [H] 
Fri Aug 07, 3:30pm EDT 
Cailan Li Category O: Methods (Part II) We begin by answering all your questions. We then introduce standard filtrations on modules in Category \(\mathcal{O}\), show basic properties and sketch how the multiplicities of a Verma module in a standard filtration of a module is related to the duality functor. We will then explain some technical details about \(\rho\)dominant weights and finally show Category \(\mathcal{O}\) has some projectives. Notes: here (PDF) References: [H] 
Fri Aug 14  No seminar (due to QUACKS) 
Fri Aug 21 
JinCheng Guu Projectives in Category \(\mathcal{O}\)
References: [H] 
Fri Aug 28 
Mrudul Thatte Verma's Thesis We will introduce contravariant forms and establish their existence and uniqueness (up to scaling) on highest weight modules. Then we will discuss various features of Verma modules including: simple submodules, homomorphisms, simplicity criterion, embeddings. This will help us analyze the composition factors of Verma modules and allow us to decompose Category \(\mathcal{O}\) into blocks indexed by \(\rho\)antidominant weights. Time permitting, we will discuss how an error in Verma's thesis implies that all composition factors appear with multiplicity 1. Notes: here (PDF) References: [H] 
Fri Sep 04 
Álvaro Martínez Jantzen's Thesis The embeddings \(M(s_\alpha\bullet \lambda) \hookrightarrow M(\lambda)\) from last time give us sufficient conditions for \([M(\lambda):L(\mu)]\neq 0\). We will start by stating the BGG Theorem, which determines exactly for which \(\mu\) this multiplicity is nonzero. This will be motivation for Jantzen’s Filtration Theorem, of which the BGG Theorem is a corollary. As another application, we will determine the exact multiplicities for the \(\mathfrak{sl}_3(\mathbb{C})\) case for all \(\lambda\) integral and dotregular. Finally we will prove Jantzen’s Filtration Theorem modulo Shapovalov’s formula. Notes: here (PDF) References: [H] 
Fri Sep 11 
Micah Gay BGG resolution This week we will discuss topics relating to BGG resolutions, including:
References: [H] 
Fri Sep 18 
Nikolay Grantcharov Translation functors in category O Translation functors are defined by tensoring with a finite dimensional \(\mathfrak{g}\)representation and then projecting to a block. Our goal is to describe how these functors act on integral blocks. We show they provide an equivalence of categories between all regular integral blocks, and then describe the result of translating from regular integral blocks to blocks parameterized by weights lying on walls. Notes: here (PDF) References: [H] 
Fri Sep 27  No seminar (break) 
Fri Oct 02  No seminar (break) 
Fri Oct 09 
JinCheng Guu The Kazhdan Lusztig Conjectures We've been seeing two important building blocks of the category \(\mathcal{O}\), namely the Verma modules \(M\) and the simple modules \(L\). In the Grothedieck ring, both of them form a basis, so we naturally want to know how the change of basis matrix looks like. It turns out that the entries can be expressed in terms of a set of mysterious polynomials, the KazhdanLusztig polynomials. In this talk, we will see what they are, and sketch a proof of the fact above. Notes: here (PDF) References: Yi Sun's notes Perverse sheaves and the KazhdanLusztig conjectures 
Fri Oct 16 
Cailan Li What makes a Sheaf Perverse, Part II We begin by reviewing some notions from Part I and then give the definition of a perverse sheaf (for real this time). After giving some examples, we then introduce BottSamelson varieties and show how using the decomposition theorem, you too can categorify the Hecke Algebra. Time permmiting, we might say a few words on Lusztig's canonical basis. Notes: Part 1 (PDF), Part 2 (PDF) References:

Fri Oct 23 
Henry Liu Category O for quantum affine algebras This will be an overview of some known aspects of the representation theory of quantum affine algebras, with \(U_q(\hat{\mathfrak{sl}}_2)\) as the main (but certainly not representative) example. I'll try to explain how cluster algebras show up in this setting — another example of categorification. Notes: here (PDF) References:

Fri Oct 30 
Álvaro Martínez The char \(p\) story We will motivate the (in many ways, parallel) study of representations of reductive algebraic groups over characteristic \(p\) and describe some of its main features, as well as their connection to the modular representation theory of finite groups. The main examples will be \(\mathrm{SL}_2\) and \(\mathrm{SL}_3\). Notes: here (PDF) References: in last slide of notes. 
Fri Nov 06 
Mikhail Khovanov Category O and categorification of Lie algebra representations We'll go over an old paper by Bernstein, I. Frenkel and Khovanov on categorification of commuting actions of \(\mathfrak{sl}(2)\) and the TemperleyLieb algebra via projective and Zuckerman functors on maximal parabolic and singular blocks of category O for \(\mathfrak{gl}(n)\). We'll also explain how to view these constructions in the modern setup, 20 years later. Notes: here (PDF) References: [BFK00] 
Fri Nov 13 
Mikhail Khovanov Category O and categorification of Lie algebra representations (part 2) We'll go over an old paper by Bernstein, I. Frenkel and Khovanov on categorification of commuting actions of \(\mathfrak{sl}(2)\) and the TemperleyLieb algebra via projective and Zuckerman functors on maximal parabolic and singular blocks of category O for \(\mathfrak{gl}(n)\). We'll also explain how to view these constructions in the modern setup, 20 years later. Notes: here (PDF) References: [BFK00] 
Sun Nov 22, 11:40am ET 
Maithreya Sitaraman Three perspectives on BorelWeilBott BorelWeilBott can be approached from three very different perspectives: (1) the algebraic geometric perspective (2) The category O perspective and (3) The differential geometric (ChevalleyEilenberg) perspective. In this talk, we will examine how all these different perspectives are used to deduce the same thing, and we will thereby appreciate the interconnectedness of mathematics. Notes: TBA References: TBA 
Fri Nov 27  No seminar (Thanksgiving) 
Fri Dec 04 
Henry Liu More on quantum affine algebras Abstract: TBA Notes: TBA References: TBA 
Fri Dec 11 
Mikhail Khovanov TBA Abstract: TBA Notes: TBA References: TBA 