by B. L. Feigin
I
I saw Fuchs for the first time in 1969. I liked mathematics; I wanted to study it. At least in my case, the choice had already been made, even if unconsciously. In other words, “mathematics is for me, it is that world where I would like to be.” The problem was that it was absolutely unclear what it meant to do mathematics. There were many research seminars at Mekhmat and apparently they were the gates to this “brave new world”. Fuchs was an active participant in at least two of them, Gelfand’s and Arnold’s. I started coming to both of them when I was a first-year student. Fuchs is a big, physically strong man; people told stories about his strength. I heard from Andryusha Zelevinsky that he could shift a row of desks with one movement of his arm. At these seminars he was the expert in topology and Gelfand, for example, always asked him whenever a problem in algebraic topology arose — and such problems did arise because discussed at the seminar were various forms of the index theorem, works of Novikov, and much more, for example, algebraic K-theory, although this was a little later.
In order to begin doing mathematics, one has to jostle around, try this and that, make sense of books and papers. All of this is important, but in doing this it is very easy to get lost and despair. It is as if you have found yourself inside a forest with a myriad of entangled paths that lead no one knows where. This is why it is so important to have somebody to talk to. Somebody must show by example how to walk these paths and what it means to do mathematics.
Among participants of the Gelfand and Arnold seminars were many remarkable mathematicians of various ages; suffice it to mention such names as Dima Kazhdan, Seryozha Gelfand, Gabrielov, Gusein-Zade… They were young back then, but somewhat older mathematicians were there, too — Arnold, Novikov, Kirillov, Fuchs — as well as more senior people: Shilov, Gindikin, Karpelevich, Minlos, Kostyuchenko. My problem was that I was afraid of these people; in my eyes they were (to some extent) Olympians. It was hard even to come closer to them, much less talk to them. In my case the problem was solved by Gelfand. Fuchs had a reputation — quite deservedly — of being the kindest person at the seminar. Gelfand clearly distinguished Fuchs from the others. Izrail Moiseevich’s way of dealing with speakers was unusual: he could interrupt, he could be unpleasant. But Dmitry Borisovich was excepted; everybody loved him, Gelfand loved him, too, and it was Gelfand who asked Fuchs to become my adviser.
This idea of Gelfand’s proved to be very successful. Fuchs helped me (not immediately of course) to overcome my lack of confidence, and this, I think, is the adviser’s main task. We worked together for a long time, and this was very important for me and, I believe, for him, too.
Our work had to do with various topics, but mostly we studied representation theory of the Virasoro algebra.
Here I will talk about our work, but this won’t be a review of published papers; rather this is an essay on how Gelfand–Fuchs theory of cohomology of Lie algebras of vector fields on manifolds naturally developed into what later became known as conformal field theory. Dmitry Borisovich worked a lot with Gelfand and later with me, but this was not a change of subject. It turned out (although it was not clear at the time) that representation theory of the Virasoro algebra logically follows Gelfand–Fuchs theory. Of course the story I’ll tell will be very incomplete.
I would also like to write about how we worked together. I think we were friends, as far as possible (Dmitry Borisovich is 14 years my senior). I went to his apartment on 13th Parkovaya Street once a week. We worked in a one-room apartment on the first floor, and in the apartment next door lived his mother; there we had lunch. The meal usually consisted of a cutlet with a side of buckwheat kasha invariably followed by cold coffee. Summers I spent in a dacha with my family. The Fuchs dacha was nearby and I cycled there, just as Fuchs sometimes cycled to my place when in Moscow. It so happened that while we were staying at the dacha, my second son Zhenya was born in the maternity hospital of the nearby town of Zhukovskiǐ. My wife was still there when Fuchs came, and along with Serezha Tabachnikov, who was visiting us, we went to the maternity hospital and stood outside. Visits were not allowed, nor would they take anything from the visitors, but the moms would drop a rope from the window, the parcel would be fastened to the end of the rope and then lifted back into the room. We did this, too.
I remember how it occurred to me that one could use the modules of semi-infinite forms in order to prove the Kac determinant formula (that is, to find the values of the parameters for which the Verma module over the Virasoro algebra is reducible). The very idea that the modules of semi-infinite forms are at all relevant naturally arose from thinking over the results by Fuchs and myself about antisymmetric invariant differential operators on the line. I was traveling to Chernogolovka,1 but I was so excited that I stopped by the Fuchs’ place. The idea was raw but it was clear it was bound to work. When I visited him next, in his mind was the understanding of what I had said along with a whole host of other things to do. Thus our work on the Virasoro algebra began.
I learnt a lot from Fuchs. He is very modest, kindly, generous. He shared everything he knew, conceived, understood. Now I realize that he did have difficulties, personal and mathematical, but to me he seemed to be the ideal human being. So I thought back then and all in all I still think so.
II
The continuous cohomology of Lie algebras of vector fields on manifolds (Gelfand–Fuchs theory) is what formed me as a mathematician, made an extraordinarily strong emotional impact on me. I think everybody experiences such a watershed moment: you pass it, and now you are a mathematician. I studied different things, I marveled at some of them. I was struck by the main idea of homological algebra, by much from algebraic geometry. I liked very much class field theory. However, coming to grips with things new to me (and especially with class field theory) was painful and difficult. I did not despair, but everything was covered in fog and, what is more important, different things remarkable in their own right would not form a whole.
Everything changed when I tried to delve into continuous cohomology theory. For me it is (to this day) an example of an “ideal” mathematical theory. On the one hand, there is its remarkable beauty, clarity, and logic. On the other, there is its “minimalism”, the absence of unnecessary entities.2 In other words, an ideal theory is a compact world; it lives and develops according to its own laws, but it is “open”. Its openness means that such a theory is like that droplet which reflects the entire world. This is the point of good minimalist music. And another thing: an ideal theory, perhaps because of its compactness, carries a charge, an energy bomb as it were, a powerful source of motion.
And so I was shaken, and what happened to me can be considered an epiphany. This was the result of my attempts to understand this theory, but also — and of course to a much greater degree — because of talking with Dmitry Borisovich.
I was lucky, of course. It is good to enter mathematics through an area which is “compact”; this way it is easier to find a problem, to see what you like. Algebraic geometry is frighteningly large; it is so much harder. However, as I shall try to explain, the compactness of Gelfand–Fuchs theory is illusory, and in fact it extends to infinity.
The continuous cohomology of Lie algebras of vector fields on manifolds implicitly (sometimes explicitly, too) contains the rudiments of many other mathematical theories, and I shall try to tell you about a part of all of this. What I shall tell is my own point of view; some of it I understood on my own, some became clear as a result of joint work with Dmitry Borisovich.
Cyclic cohomology or additive -theory
Vector fields are infinitesimal symmetries of manifolds, and an analogue
that immediately comes to mind is a current algebra. These are algebras of
infinitesimal symmetries of bundles. Let
Let
The local problem in
Gelfand–Fuchs theory is to compute the continuous
cohomology of the Lie algebra of formal vector fields on of and invariants are labeled by some graphs. Thus one obtains a
complex with a basis whose elements are labeled by graphs of special
type: the
People have managed to apply these ideas to current algebras in two
cases. The first is the one where
Gelfand and Fuchs tried to compute the cohomology of the Lie algebra of Hamiltonian vector fields. The local problem also leads to a graph-complex, and to be looked after are the symplectic Lie algebra invariants. The complex they obtained was very involved, and no wonder, because if the dimension tends to infinity, then it becomes the famous graph-complex. The reason for this phenomenon remains mysterious even now, although a partial explanation can be found in Rozansky–Witten theory. (A relation to Gelfand–Fuchs theory and characteristic classes of foliations was recognized by Kapranov and Kontsevich soon after the Rozansky–Witten paper appeared.)
The second case where the cohomology of in other words, the algebra is the
ring of regular functions on the line with values in a finite-dimensional
semisimple Lie algebra. The methods used are quite different. This
algebra is a Lie subalgebra of
Continuous cohomology of loop algebras
The case of
All of this can be applied to
Gelfand and Fuchs worked on a rather concrete problem:
they analyzed the
standard complex of the Lie algebra of vector fields on a manifold. (They
started with the case of trivial coefficients, but later considered
modules of tensor fields.) They used methods of functional analysis,
specifically the theory of distributions, an object of Gelfand’s
fascination in those times. But fundamentally, the essence of their
work was purely
algebraic. Among other things, Gelfand and Fuchs constructed a system of
sheaves with connection on the configuration space of collections of points
on a manifold. This system of sheaves enjoys a factorization property. The
fiber over a point is a standard complex of the Lie algebra
of vector fields over the formal disc centered at this point; over a
finite collection of points, the fiber is the tensor product of fibers
over each point. In modern language, this means
Gelfand and Fuchs
constructed an
The Virasoro algebra
The Virasoro algebra is the (universal) central extension of the Lie algebra of vector fields on the circle. The Gelfand–Fuchs computation showed that the cohomology of this Lie algebra is the superpolynomial ring on two generators, one in dimension 2, another in dimension 3; thus the Virasoro algebra was born. Another source is string theory, where the Virasoro algebra is the main actor.
It was clear from the beginning that the Virasoro algebra is similar
to the central extension
of The latter’s central
extension is a Kac–Moody Lie algebra, and is, therefore, a subject
with a well-developed theory. There are a lot of tools to study representations
of Kac–Moody Lie algebras, algebraic and geometric. The Virasoro algebra
also has a category
of highest-weight modules, there are Verma modules, and all the usual
questions can be asked. The very first question is this: The Verma modules
The methods of Kac–Moody algebra theory, however, do not directly work in the Virasoro algebra case. There are many reasons for this, e.g., there is no Casimir element. Additionally, there is no obvious way to define integrable representations. Such representations are well known in the Kac–Moody case; they are analogues of finite-dimensional modules over a semisimple Lie algebra. The existence of finite-dimensional modules greatly facilitates the study of the category of highest-weight modules.
Fuchs and I did a lot of work on representations of the Virasoro algebra. We managed to prove the Kac conjecture, using the relation with invariant differential operators on the line. We analyzed the structure of Verma modules, obtained a lot of information on highest-weight modules. The creators of conformal field theory, Belavin, Polyakov, and Zamolodchikov, introduced the concept of a minimal model. We shall say more about these models a little later, but their essential part is a special class of modules over the Virasoro algebra. It turned out that these modules are a proper analogue of integrable representations. Fuchs and I constructed the Bernstein–Gelfand–Gelfand-type resolutions of these modules, and in particular obtained the Virasoro algebra versions of the Weyl character formula. I shall talk more about the methods used a little later, as these methods belong in conformal field theory. The Virasoro algebra and loop algebras are particular examples of what is known as a vertex algebra. These are objects of a theory that has been rapidly developing for over 30 years with no end in sight. Vertex algebras also enjoy the existence of minimal models, integrable representations, resolutions, character formulas, etc. One cannot say that this activity grew out of the Gelfand–Fuchs cohomology; nevertheless there are close connections.
Conformal field theory
Vertex algebras are a piece (an essential one) of the mathematical machinery of conformal field theory. Let us point out that the Virasoro algebra is the algebra of conformal symmetries (local and infinitesimal ones), and therefore it is a key.
Conformal field theory originated in works of Belavin, Polyakov and Zamolodchikov, as well as Friedan and Shenker, and its consequences are innumerable and hard to process. This theory cannot be called “ideal”; it does not satisfy the “minimalistic” principles, and yet it has some of this flavor too. Conformal field theory arose inside quantum field theory as its special case. Quantum field theory per se is the very negation of the principle of minimizing the number of entities. Before us is a world without boundary, a tower of Babel, to which turrets are added all the time, parts of the construction get abandoned, other parts collapse or sit in neglect; meanwhile the main aim of reaching heaven is not coming much closer. Conformal field theory is an outgrowth of quantum field theory obtained by means of “cutting off the ends”; that is to say, they extracted that part which is supported by a hard mathematical frame. It turned out that this piece had acquired (or perhaps accumulated) energy that led to an explosive growth of a considerable chunk of mathematics, and maybe physics, too. Furthermore, parts of physics and mathematics began to fuse into a whole, and all this was happening right in front of us.
Very much a bird’s eye view on the conformal field theory structure
is as follows. To begin with, the theory is 2-dimensional; in other
words, it lives on a Riemann surface, a 2-dimensional manifold with
metric. It possesses fields, which are attached to points. Furthermore,
assigned to a point is in fact an entire space of states. For example,
a quantum-mechanical description of an electron involves a 2-dimensional
space of states, which carries an action of the orthogonal group; the
latter is understood as the symmetry group. In conformal field theory,
the symmetry group is bigger; it comprises the set of all (infinitesimal)
transformations of a punctured disc — or rather the central extension
of this Lie algebra,
the Virasoro algebra. Therefore, the space of states
The main attribute of such functionals is
the operation of “fusion”: it describes what happens to the spaces
Fuchs and I spent many years working on conformal field theory. Nevertheless, it was very difficult to understand what was happening there; we were hampered by the physics language, completely unfamiliar and frightening. Things became much easier when I understood that many structure elements of conformal field theory are not new to me: they had already appeared in continuous cohomology theory (and the theory of modular forms.)
In addition to the concept of a correlation function there is a related
concept of a modular functor. As an example, let us consider what the
founders called a minimal model or (more technically) a rational conformal
theory. According to
Segal
and
Atiyah,
a modular functor is a version of cohomology theory, or something that
is called a 3-dimensional field theory. Namely, there is a rule that
to each closed 2-dimensional manifold
Fuchs and I, unknowingly, studied the modular functor that arises via
minimal models. We worked on the following problem.
Let
As I have already pointed out, the Virasoro algebra and the affine Lie
algebras are close relatives and are the main examples of vertex operator
algebras. Fuchs and I studied the Virasoro algebra representations using
its embedding into simpler vertex algebras, Heisenberg and Clifford.
We conceived of this method by ourselves, but from the conformal field
theory perspective this is an infinitely natural thing to do. Conformal
field theories per se are hard to analyze, people try to embed them into
simpler ones, those we know enough about, and especially those where we can
find correlation functions. This approach is called bosonization. Since all
of this had worked in the case of the Virasoro algebra, it seemed natural
to try and do the same thing for the affine Lie algebras. Bosonization of
by for example, a homogeneous
space. An algebra of differential operators can be twisted, meaning that
it acts not on functions but on sections of a
In the affine Lie algebra case such approach works too, because an affine
Lie algebra has its flag manifold and can, therefore, be mapped into
the corresponding algebra of differential operators. Furthermore, in
this case there are several inequivalent flag manifolds,
and hence several
bosonizations. In fact, in the affine Lie algebra case there are flag
manifolds of
three types: thick, thin, and semi-infinite. The
thin one resembles the finite-dimensional flag manifold the most, and is
in fact an inductive limit of finite-dimensional ones. The thick one is
more of a projective limit, and is built of infinite-dimensional cells
of finite codimension,
For the Virasoro algebra, as well as for most vertex algebras, a construction resembling thick and thin flags is yet to be found, but the known bosonizations are semi-infinite constructions in spirit.
Therefore, a considerable chunk of modern theory originated (partially,
of course) in our work on the Virasoro algebra representation theory. In
a sense, we extracted the subcategory generated by the Verma modules,
which is akin to the category of
But this is not it, the situation turned out to be even more interesting.
There is no thick flag manifold for the Virasoro algebra, but there is an
important analogue. Namely, the “thick flag manifold” for the affine
Lie algebra
In my talk at ICM-90 in Kyoto I proposed an interpretation of the
modular functor in the language of
Gelfand–Fuchs theory. Namely,
let
The homology of
If the 1-dimensional representation corresponds
to
Integral representations and screening operators
Integral representations of correlation functions naturally arise
in our works. We were trying to find explicit formulas for singular
vectors in Verma modules over the Virasoro algebra or,
equivalently, describe homomorphisms between Verma modules. This is an
important problem and, unfortunately, there are no “good” formulas
in the case of Kac–Moody
Lie algebras. In the Virasoro case what helps
is bosonization. Namely, the Virasoro algebra embeds into the universal
enveloping algebra of the Heisenberg Lie algebra:
Intertwining operators between Fock modules are constructed in the form
of integrals of vertex operators and are called a screening.
The vertex
operator
Screening operators and the related integral representations find
numerous applications in conformal field theory. One of the important
such applications is known as a “topological” realization of the
modular functor. As we have seen, the modular functor that comes from the
minimal models assigns to an algebraic curve the space of coinvariants
of the vacuum module model.) The details can be found in the book by
Bezrukavnikov,
Finkelberg,
and
Schechtman.
In the series of works by Schechtman and Varchenko integral representations of solutions for Knizhnik–Zamolodchikov equations were found. Their results can also be obtained using the technique of screening operators.
It is also important to note the following. The Virasoro algebra has a
generalization, the
The algebra and Goncharova’s theorem
I would like to write about the work on the algebra
Here
Lida Goncharova was a student of Dmitry Borisovich; she was an extraordinary human being and a gifted mathematician. She married Sasha Geronimus, a good mathematician, who later became a very well known Russian Orthodox priest. Lida became a priest’s wife and quit mathematics.3
In order to use the Goncharova spectral sequence, one needs to know
The standard chain complex of
The Euler identity related to
A somewhat different method of dealing with the standard complex uses a natural positive definite form on the maximal nilpotent subalgebra. This form extends to the complex, and allows one to define the Laplace operator. Its kernel is the homology. The spectral problem for the Laplace operator is easy to solve for Kac–Moody Lie algebras, but in the Virasoro case it is nontrivial, and was solved by Gelfand, Fuchs and myself [10]. This paper contains an inaccuracy, which was corrected by Felix Vainstein.
I would like to note that the information on the homology of
III
I would like to add a few words to what I wrote at the beginning. Fuchs played a defining role in my development as a mathematician; his influence was more important for me than that of Gelfand’s school, to which we both belong. Yes, it is difficult to become a mathematician; it is especially difficult to realize what the subject of mathematics is, what is it in mathematics that is yours, that you like. Gelfand used to say that a mathematician always works on the same subject. This means that a human being likes some things, and there cannot be too many such things, because the capacity of a human mind is limited. When applied to Gelfand himself, these words sound paradoxical as he seemed to change his research area many times. Nevertheless, he thought he had always worked on the same subject.
Very well, a mathematician has decided what they like, has developed a degree of confidence. In other words, a transition from childhood to adulthood has taken place, even if incompletely. But adulthood creates new problems. I will write about some of them, those that were especially important for me.
I graduated from Moscow State University in 1974. My undergraduate thesis was a piece of relatively original research, a continuation of a joint work by Gelfand, Fuchs and myself on deformations of characteristic classes of foliations. Going to graduate school, in my case, was not an option (because Jews were not admitted), and I had to do with my life something else. I found a job in a computer center and had to figure out how to combine my job with mathematical research. I must admit I was quite lost. It was a difficult situation where many people would quit mathematics. I did not want this at all, but life had changed, I had to spend days at work. Abandoning mathematics was for me unacceptable, but it was unclear how to organize my life. Fuchs told me that my situation was not unusual, that many people were in the same position. I understood that it was up to me to solve my problems; that is, I had to find a way to work, think, talk with other mathematicians. Fuchs told me that it would be difficult and offered to continue staying in touch. This meant coming to his place once a week and not being distressed if no new thoughts have occurred to me since the previous visit. He thought it was essential to hold out for the time being and then the life would settle down and I would regain the ability to think and work. Fuchs had experience, he knew how it was with other people, and he knew something about me. His support and help during this difficult time was not merely important, without it I would have been unable to overcome all of this. To say that I am grateful to Fuchs is to say nothing.
Life went on; after the computer center I found myself at the graduate school of the Yaroslavl University; later I defended my Ph.D. thesis in Leningrad. Thesis defense was a complicated process in those days and without Fuchs’s help it was unlikely to work out. I married, which of course did not make things any easier, but the very idea — keep on doing mathematics no matter what — remained, and without Fuchs this would have been impossible.
All right then, I became a mathematician, I had written papers, I had obtained some new results. One common problem is this: how do you treat the fact that other people work on the same stuff, use what you have done. This problem is multifaceted; here we will not be able to discuss it in earnest. It is clear, however, that everyone competes with other people (as in sports). It is inevitable that you have to decide what to do with this circumstance. As with many other things, there is no satisfactory solution here. One extreme is when you ignore the public opinion, write very few papers, stick to your course. Some people do that, but it is very difficult and often counterproductive. Mathematics is a collective endeavor and it is very important to understand that you are a part of a community that is involved in a certain process (the process of gaining knowledge, understanding, it is not very important what you call this process.) Practice shows that another extreme — the complete adherence to conventions, becoming an active part of the system — is also counterproductive. One must learn how to remain true to oneself, and I have learnt this from Fuchs, if at all. True, it is impossible not to get perturbed because some people are better than you at some things, that your work is not mentioned; furthermore, it is human nature that you cannot help envying those who are smarter, more successful than you. Nevertheless, it is important to develop an ability to live with all of this — yes, those feelings are there, but they are at the background, as it were. The main thing — the rules according to which you work and live — they are a step above. In other words, torments and tribulations are unavoidable, but you have to learn how to deal with them. My solution to these problems is not ideal but, especially now, I realize that I largely followed Fuchs. We are of course very different; nevertheless it was his way of solving these questions that I applied to my life. Note that at no time did Fuchs explicitly state any kind of moral principle, but as a little child I formed my behavior, my principles imitating the grown-ups.
I have already said what was important to say. Nevertheless, I would like to touch upon one more question. Mathematics is not only a way to express yourself and realize your abilities. Psychologically, mathematics is perceived as a process of gaining insight into a remarkably beautiful entity, and this process is not instantaneous, rather it is extended in time. In other words, somebody already did this before us — admired this entity and tried to understand it — and somebody will carry on after us. It is difficult, if not outright impossible, to explain this with at least some semblance of clarity, but practically any mathematician has a hunch that this must be so. As a practical matter, this means that you received something from your teachers, and your task is to pass this along. To do so is sometimes an obvious necessity. I know people who can properly function only when surrounded by pupils. Therefore, teaching and working with students is an essential part of mathematical life. I have had quite a few students; my relationships with them evolved in many different ways, they were sometimes difficult, but here I also learnt a lot from Fuchs. He spent much of his time on me, and to a large extent I project his way of interaction. I received from Fuchs a certain “knowledge” about mathematics and to the extent I can I pass it along.
IV
The short note [1] is the first Gelfand–Fuchs paper on continuous cohomology. This is not their first joint paper; the previous ones are preparatory, and among other things they contain a discussion of continuous group cohomology. The paper [2] contains all the main ideas; its second part is [3]. The paper [5], joint with D. I. Kalinin, is devoted to the cohomology of the Lie algebra of Hamiltonian vector fields.
The very important paper [4], joint with D. A. Kazhdan, appeared in 1972. Introduced in it is the key (to me) concept of a principal homogeneous space of an infinite-dimensional Lie algebra, such as the Lie algebra of formal vector fields. They introduced techniques for dealing with formal geometry, and these found numerous applications; in particular, they make much clearer “local” proofs of the index theorem and its applications.
Fuchs’ 1973 study [6] is an interesting review paper on characteristic classes of foliations; [9] is a more extended review of the same subject.
My first paper with Gelfand and Fuchs [8] deals with variations of characteristic classes of foliations. Around that time I was greatly impressed by the Fuchs paper “Quillenization and bordisms” [7], an excellent introduction to K-theory (at least for me.)
The work by Fuchs and myself on the Virasoro algebra is put together (incompletely) in the big review paper [15]. This paper appeared in 1990, but it had been written a year or two earlier. The Soviet Union was coming to an end, and unfortunately at about the same time our collaboration ended too. Times changed, and many people, including Gelfand and Fuchs, moved to the United States. The beginning of our joint work is the paper [11] devoted to invariant differential operators on the line. There was no Virasoro algebra there yet, but it made its appearance in [12]. The very fact that the problem on invariant differential operators is related to the semi-infinite construction of Virasoro algebra modules allowed us both to prove our conjectures on the classification of invariant skew-symmetric differential operators and to prove the Kac conjecture about the Virasoro algebra Verma modules.
The paper [13] studies the structure of the Virasoro algebra Verma modules and constructs the Bernstein–Gelfand–Gelfand-type resolutions, which imply the character formulas of those modules that arise in minimal models. All of this is described in greater detail in the aforementioned [15].
This list of our joint works is by no means complete; we also wrote two papers joint with Gelfand.
I would like to add a few words about “the sources and component parts”4
of our work. I have already written about one source,
cohomology theory
of Lie algebras. This is what we worked on at the beginning. We mostly
restricted ourselves to the 1-dimensional case, because much more can be
understood there. Furthermore, we investigated the
zeroth (co-)homology with
values in tensor products of modules of tensor fields on the line, which
is very close to the problem about classification of invariant differential
operators. The second “source” of our work is
representation theory
of finite-dimensional and affine Lie algebras, which we tried to emulate
by studying, in as much detail as we could, the Verma module structure,
explicit formulas for intertwining operators and singular vectors, as well as
Bernstein–Gelfand–Gelfand-type resolutions. The problem about coinvariants
arose as an attempt to understand an analogue of
the localization functor for
Kac–Moody Lie algebras. But we began not even here, we wanted to understand
what happens to singular vectors under quotienting out by a submodule. For
example, a Verma module can be quotiented out by
the subalgebra generated by
V
I would like to conclude by saying a few words about Fuchs the teacher. I was an object of Fuchs’s efforts as a pedagogue. These efforts were very indirect, he did not teach me anything, as it were, but I followed him around like a puppy, and nothing could be better for me than this. I could not attend his famous lectures on homotopical topology, but his textbook on this subject (with Fomenko and Gutenmacher) was perhaps the only textbook I ever read almost to the end. It is extremely well written, and algebraic topology is the first mathematical theory I learned. All the others were much easier as I had already gotten one. Fuchs is the author of a number of textbooks; I like the one on cohomology of Lie algebras [14], a remarkable, clearly written book.