If you have problems downloading any of these PDF files, please email me at: arkadiy@uoregon.edu
Monomial bialgebras (with J. Greenstein and J.-R. Li),
Starting from a single solution of QYBE (or CYBE) we produce an infinite family of solutions of QYBE (or CYBE) parametrized by transitive arrays and, in particular, by signed permutations. We are especially interested in cases when such solutions yield quasi-triangular structures on direct powers of Lie bialgebras and tensor powers of Hopf algebras. We obtain infinite families of such structures as well and study the corresponding Poisson-Lie structures and co-quasi-triangular algebras.
Noncommutative marked surfaces II: tagged triangulations, clusters, and their symmetries (with V. Retakh and M. Huang),
The aim of the paper is to define noncommutative cluster structure on several algebras A related to marked surfaces possibly with orbifold points of various orders, which includes noncommutative clusters, i.e., embeddings of a given group G into the multiplicative monoid A× and an action of a certain braid-like group BrA by automorphisms of each cluster group in a compatible way. For punctured surfaces we construct new symmetries, noncommutative tagged clusters and establish a noncommutative Laurent Phenomenon.
Multiple Horn problems for planar networks and invertible matrices (with A. Alekseev, A. Gurenkova, Y. Li), Advances in Mathematics, Vol. 478, 2025.
The multiplicative multiple Horn problem is asking to determine possible singular values of the combinations AB, BC and ABC for a triple of invertible matrices A,B,C with given singular values. There are similar problems for eigenvalues of sums of Hermitian matrices (the additive problem), and for maximal weights of multi-paths in concatenations of planar networks (the tropical problem). For the planar network multiple Horn problem, we establish necessary conditions, and we conjecture that for large enough networks they are also sufficient. These conditions are given by the trace equalities and rhombus inequalities (familiar from the hive description of the classical Horn problem), and by the new set of tetrahedron equalities. Furthermore, if one imposes Gelfand-Zeitlin conditions on weights of planar networks, tetrahedron equalities turn into the octahedron recurrence from the theory of crystals. We give a geometric interpretation of our results in terms of positive varieties with potential. In this approach, rhombus inequalities follow from the inequality Φt≤0 for the tropicalized potential, and tetrahedron equalities are obtained as tropicalization of certain Plücker relations. For the multiplicative problem, we introduce a scaling parameter s, and we show that for s large enough (corresponding to exponentially large/small singular values) the Duistermaat-Heckman measure associated to the multiplicative problem concentrates in a small neighborhood of the octahedron recurrence locus.
Hecke and Artin monoids and their homomorphisms (with J. Greenstein and J.-R. Li), submitted.
This work was motivated by a striking observation that parabolic projections of Hecke monoids map respect all parabolic elements. We found other classes of homomorphisms of Hecke monoids with the same property and discovered that many of them lift to homomorphisms of covering Artin monoids with a similar property. It turned out that they belong to a much larger class (in fact, a category) of homomorphisms of Artin monoids, most of which appear to be new.
Generalized electrical Lie algebras (with A. Gainutdinov, V. Gorbunov), Advances in Mathematics, Vol. 478, 2025.
We generalize the electrical Lie algebras originally introduced by Lam and Pylyavskyy in several ways. To each Kac-Moody Lie algebra g we associate two types (vertex type and edge type) of the generalized electrical algebras. The electrical Lie algebras of vertex type are always subalgebras of g and are flat deformations of the nilpotent Lie subalgebra of g. In many cases including sln, son, and sp2n we find new (edge) models for our generalized electrical Lie algebras of vertex type. Finding an edge model in general is an interesting an open problem.
Valuations, bijections, and bases (with D. Grigoriev), submitted.
The aim of this paper is to build a theory of commutative and noncommutative injective valuations of various algebras (including algebras with zero divisors). The targets of our valuations are (well-)ordered commutative and noncommutative (partial or entire) semigroups including any sub-semigroups of the free monoid Fn on n generators and various quotients. In the case when the (partial) valuation semigroup is finitely generated, we construct a generalization of the standard monomial bases for the so-valued algebra, which seems to be new in noncommutative case. Quite remarkably, for any pair of well-ordered valuations one has canonical bijections between the valuation semigroups, which serve as analogs of the celebrated Jordan-Hölder correspondences and these bijections are “almost” homomorphisms of the involved (partial and entire) semigroups.
A spectacular demonstration of this remarkable property of JH-bijections for quantum Schubert cells A=Uq(w) results in mysterious "symplectomorphisms" of involved skew symmetric forms.
Transitive and Gallai colorings of the complete graphs (with R. M. Adin , J. Greenstein, J-R Li, A. Marmor, Y. Roichman), European Journal of Combinatorics, Vol. 130, 2025 (the journal version is different from the ArXiv one).
Gallai coloring of the complete graph is an edge-coloring with no rainbow triangle. This concept first appeared in the study of comparability graphs and anti-Ramsey theory. We introduce a transitive analogue for acyclic directed graphs, and generalize both notions to Coxeter systems, matroids and commutative algebras. It is shown that for any finite matroid (or oriented matroid), the maximal number of colors is equal to the matroid rank. This generalizes a result of Erdős-Simonovits-Sós for complete graphs. The number of Gallai (or transitive) colorings of the matroid that use at most k colors is a polynomial in k. Also, for any acyclic oriented matroid, represented over the real numbers, the number of transitive colorings using at most 2 colors is equal to the number of chambers in the dual hyperplane arrangement. We count Gallai and transitive colorings of the root system of type A using the maximal number of colors, and show that, when equipped with a natural descent set map, the resulting quasisymmetric function is symmetric and Schur-positive.
Twists on rational Cherednik algebras (with Y. Bazlov, E. Jones-Healey, A. McGaw), Quarterly Journal of Mathematics, 74 (2), 2022.
We show that braided Cherednik algebras introduced by the first two authors are cocycle twists of rational Cherednik algebras of the imprimitive complex reflection groups G(m,p,n), when m is even. This gives a new construction of mystic reflection groups which have Artin-Schelter regular rings of quantum polynomial invariants. As an application of this result, we show that a braided Cherednik algebra has a finite-dimensional representation if and only if its rational counterpart has one.
Symplectic groups over noncommutative algebras (with D. Alessandrini, V. Retakh, E. Rogozinnikov, A. Wienhard) Selecta Mathematica, 28, 82 (2022)
We introduce the symplectic group Sp2(A,σ) over a noncommutative algebra A with an anti-involution σ. We realize several classical Lie groups as Sp2 over various noncommutative algebras, which provide new insights into their structure theory. We construct several geometric spaces, on which the groups Sp2(A,σ) act. We introduce the space of isotropic A-lines, which generalizes the projective line. We describe the action of Sp2(A,σ) on isotropic A-lines, generalize the Kashiwara-Maslov index of triples and the cross ratio of quadruples of isotropic A-lines as invariants of this action. When the algebra A is Hermitian or the complexification of a Hermitian algebra, we introduce the symmetric space XSp2(A,σ), and construct different models of this space. Applying this to classical Hermitian Lie groups of tube type (realized as Sp2(A,σ)) and their complexifications, we obtain different models of the symmetric space as noncommutative generalizations of models of the hyperbolic plane and of the three-dimensional hyperbolic space. We also provide a partial classification of Hermitian algebras in Appendix A.
Geometric multiplicities (with Y. Li), preprint
In this paper, we introduce geometric multiplicities, which are
positive varieties with potential fibered over the Cartan subgroup H of
a reductive group G. They form a monoidal category and we construct a
monoidal functor from this category to the representations of the Langlands
dual group Gv of G. Using this, we explicitly compute various
multiplicities in Gv-modules
in many ways. In particular, we recover the formulas for tensor product
multiplicities of Berenstein-Zelevinsky and generalize them in several
directions. In the case when our geometric multiplicity X is a monoid,
i.e., the corresponding Gv-module is an algebra, we expect
that in many cases, the spectrum of this algebra is an affine Gv-variety
Xv, and thus the correspondence XàXv has a flavor of both the Langlands duality and mirror
symmetry.
Langlands Duality and Poisson-Lie Duality via Cluster Theory and Tropicalization (with A. Alekseev, B. Hoffman, Y. Li) Selecta Mathematica, 27, 69 (2021)
Let G be a connected semisimple Lie group. There are two
natural duality constructions that assign to it the Langlands dual group Gv
and the Poisson-Lie dual group G*. The main result of this paper is the
following relation between these two objects: the integral cone defined by the
cluster structure and the Berenstein-Kazhdan potential on the double Bruhat
cell Gv;wo,e. is isomorphic to the
integral Bohr-Sommerfeld cone defined by the Poisson structure on the partial
tropicalization of K*⊂G* (the Poisson-Lie dual of the compact form K⊂G).
The first cone parametrizes the canonical bases of irreducible G-modules.
The corresponding points in the second cone belong to integral symplectic
leaves of the partial tropicalization labeled by the highest weight of the
representation. As a by-product of our construction, we show that symplectic
volumes of generic symplectic leaves in the partial tropicalization of K*
are equal to symplectic volumes of the corresponding coadjoint orbits in k*.
To achieve these goals, we use (Langlands dual) double cluster varieties
defined by Fock and Goncharov. These are pairs of cluster varieties whose seed
matrices are transpose to each other. There is a naturally defined isomorphism
between their tropicalizations. The isomorphism between the cones described
above is a particular instance of such an isomorphism associated to the double
Bruhat cells G wo,e ⊂ G and Gv;wo,e.
On cacti and crystals (with J.
Greenstein and J.-R.
Li) Representations
and Nilpotent Orbits of Lie Algebraic Systems: in honor of the 75th Birthday of
Tony Joseph, Progress
in Mathematics, 330, 2019.
In the present work we study actions of various groups generated
by involutions on the category Oqint(g) of integrable highest weight Uq(g)-modules
and their crystal bases for any symmetrizable Kac-Moody algebra g.
The most notable of them are the cactus group and (yet conjectural) Weyl group
action on any highest weight integrable module and its lower and upper crystal
bases. Surprisingly, some generators of cactus groups are anti-involutions of
the Gelfand-Kirillov model for Oqint(g)
closely related to the remarkable quantum twists discovered by Kimura and Oya.
Poisson structures and
potentials (with A.
Alekseev,
B. Hoffman, Y.
Li) Lie
Groups, Geometry, and Representation Theory: A Tribute to the Life and Work of
Bertram Kostant, Birkhauser, 2018.
We introduce a notion of weakly log-canonical Poisson structures on
positive varieties with potentials. Such a Poisson structure is log-canonical
up to terms dominated by the potential. To a compatible real form of a weakly
log-canonical Poisson variety we assign an integrable system on the product of
a certain real convex polyhedral cone (the tropicalization of the variety) and
a compact torus. We apply this theory to the dual Poisson-Lie group G* of a
simply-connected semisimple complex Lie group G. We define a positive structure
and potential on G* and show that the natural Poisson-Lie structure on G*
is weakly log-canonical with respect to this positive structure and potential.
For K⊂G the compact real form, we show that the real form K*⊂G*
is compatible and prove that the corresponding integrable system is defined on the
product of the decorated string cone and the compact torus of dimension 1/2(dim
G - rank G).
Noncommutative Catalan
numbers (with V.
Retakh)
Annals of
Combinatorics, Vol. 23, Issue 3–4 (2019), 527–547.
The goal of this paper is to introduce and study noncommutative Catalan numbers Cn which belong to the free Laurent polynomial algebra in n generators. Our noncommutative numbers admit interesting (commutative and noncommutative) specializations, one of them related to Garsia-Haiman (q,t)-versions, another -- to solving noncommutative quadratic equations. We also establish total positivity of the corresponding (noncommutative) Hankel matrices Hn and introduce accompanying noncommutative binomial coefficients.
Factorizable module algebras (with K. Schmidt) Int. Math. Res. Not. 2019 (21), 6711–6764 (2019).
The aim of this paper is to introduce and study a large class of g-module
algebras which we call factorizable by generalizing the Gauss factorization of
(square or rectangular) matrices. This class includes coordinate algebras of
corresponding reductive groups G, their parabolic subgroups, basic
affine spaces and many others. It turns out that tensor products of
factorizable algebras are also factorizable and it is easy to create a
factorizable algebra out of virtually any g-module algebra. We
also have quantum versions of all these constructions in the category of Uq(g)-module
algebras. Quite surprisingly, our quantum factorizable algebras are naturally
acted on by the quantized enveloping algebra Uq(g*) of
the dual Lie bialgebra g* of g.
Hecke-Hopf algebras (with D. Kazhdan) Advances
in Mathematics, Vol. 353 (2019), 312–395.
Let W
be a Coxeter group. The goal of the paper is to construct new Hopf algebras
contain Hecke algebras Hq(W) as (left) coideal subalgebras. Our Hecke-Hopf algebras
H(W) have a number of applications. In particular they
provide new solutions of quantum Yang-Baxter equation and lead to a
construction of a new family of endo-functors of the category of Hq(W)-modules. Hecke-Hopf
algebras for the symmetric group are related to Fomin-Kirillov algebras; for an
arbitrary Coxeter group W the “Demazure” part of H(W)
is being acted upon by generalized braided derivatives which generate the
corresponding (generalized) Nichols algebra.
Canonical bases of quantum Schubert cells
and their symmetries (with J. Greenstein), Selecta Mathematica, 23, pages 2755–2799
(2017).
The goal of this work
is to provide an elementary construction of the canonical basis B(w) in
each quantum Schubert cell Uq(w) and to establish its invariance
under modified Lusztig’s symmetries. To that effect, we obtain a direct
characterization of the upper global basis Bup
in terms of a suitable bilinear form and show that B(w) is
contained in Bup and its large part is
preserved by modified Lusztig’s symmetries.
Noncommutative marked surfaces (with V. Retakh), Advances in Mathematics, Vol 328 (2018), pages 1010–1087.
The aim of the paper is to attach a noncommutative cluster-like structure to each marked surface Σ. This is a noncommutative algebra AΣ generated by “noncommutative geodesics” between marked points subject to certain triangular relations and noncommutative analogues of Ptolemy-Plucker relations. It turns out that the algebra AΣ exhibits a noncommutative Laurent Phenomenon with respect to any triangulation of Σ, which confirms its “cluster nature.” As a surprising byproduct, we obtain a new topological invariant of Σ, which is a free or a 1-relator group easily computable in terms of any triangulation of Σ. Another application is the proof of Laurentness and positivity of certain discrete noncommutative integrable systems.
Generalized adjoint actions (with V. Retakh), Journal of Lie Theory, 26 (2016), No. 1, pages 219–225.
The aim of this paper is to generalize the classical formula exye-x=Σk≥0
1/k! (ad x)k(y). We also obtain combinatorial applications to q-exponentials,
q-binomials, and Hall-Littlewood polynomials.
Generalized Joseph’s
decompositions (with J. Greenstein), Comptes Rendus Mathematique, Doi :
10.1016/j.crma.2015.07.002.
We
generalize the decomposition of Uq(g) introduced by A.
Joseph and relate it, for g semisimple, to the celebrated computation of
central elements due to V. Drinfeld. In that case we construct a natural basis
in the center of Uq(g) whose elements behave as Schur
polynomials and thus explicitly identify the center with the ring of symmetric
functions.
Integrable clusters (with J. Greenstein, D. Kazhdan), Comptes Rendus Mathematique,Vol 353, 5 (2015), pages
387–390.
The goal
of this note is to study quantum clusters in which cluster variables (not
coefficients) commute which each other. It turns out that this property is
preserved by mutations. Remarkably, this is equivalent to the celebrated sign
coherence conjecture recently proved by M. Gross, P. Hacking, S. Keel and M.
Kontsevich.
Double canonical bases (with J. Greenstein), Advances in Mathematics,Vol. 316 (2017), pages 54–111.
We introduce a new class of bases for quantized universal enveloping algebras Uq(g) and other doubles attached to semisimple and Kac-Moody Lie algebras. These bases contain dual canonical bases of upper and lower halves of Uq(g) and are invariant under many symmetries including all Lusztig’s symmetries if g is semisimple. It also turns out that a part of a double canonical basis of Uq(g) spans its center.
Mystic reflection groups (with Y. Bazlov), SIGMA 10 (2014), 040, 11 pages.
This paper aims to systematically study mystic reflection groups that emerged independently in a paper by the authors and in a paper by Kirkman, Kuzmanovich, and Zhang. A detailed analysis of this class of groups reveals that they are in a nontrivial correspondence with complex reflection groups G(m,p,n). We also prove that the group algebras of corresponding groups are isomorphic and classify all such groups up to isomorphism.
Quantum cluster characters of Hall algebras (with D. Rupel), Selecta Mathematica, 21, pages 1121–1176 (2015).
The aim of the paper is to introduce a generalized quantum cluster character, which assigns to each object V of a finitary Abelian category C over a finite field Fq and any sequence i of simple objects in C the element XV,i of the corresponding algebra PC,i of q-polynomials. We prove that if C was hereditary, then the assignments VàXV,i define algebra homomorphisms from the (dual) Hall-Ringel algebra of C to the PC,i, which generalize the well-known Feigin homomorphisms from the upper half of a quantum group to q-polynomial algebras. If C is the representation category of an acyclic valued quiver (Q,d) and i=(io,io), where io is a repetition-free source-adapted sequence, then we prove that the i-character XV,i equals the quantum cluster character XV introduced earlier by the second author in [29] and [30]. Using this identification, we deduce a quantum cluster structure on the quantum unipotent cell corresponding to the square of a Coxeter element. As a corollary, we prove a conjecture from the joint paper [5] of the first author with A. Zelevinsky for such quantum unipotent cells. As a byproduct, we construct the quantum twist and prove that it preserves the triangular basis introduced by A. Zelevinsky and the first author in [6].
Cocycle twists and extensions of braided doubles (with Y. Bazlov), Contemp. Math., 592 (2013), pages 19–70.
It is well known that central extensions of a group G correspond to 2-cocycles on G. Cocycles can be used to construct extensions of G-graded algebras via a version of the Drinfeld twist introduced by Majid. We show how to define the second cohomology group of an abstract monoidal category C, generalising the Schur multiplier of a finite group and the lazy cohomology of a Hopf algebra, recently studied by Schauenburg, Bichon, Carnovale and others. A braiding on C leads to analogues of Nichols algebras in C, and we explain how the recent work on twists of Nichols algebras by Andruskiewitsch, Fantino, Garcia and Vendramin fits in our context. In the second part of the paper we propose an approach to twisting the multiplication in braided doubles, which are a class of algebras with triangular decomposition over G. Braided doubles are not G-graded, but may be embedded in a double of a Nichols algebra, where a twist is carried out. This is a source of new algebras with triangular decomposition. As an example, we show how to twist the rational Cherednik algebra of the symmetric group by the cocycle arising from the Schur covering group, obtaining the spin Cherednik algebra introduced by Wang.
Macdonald Polynomials and BGG reciprocity for current algebras (with M. Bennett, V. Chari, A. Khoroshkin, S. Loktev), Selecta Mathematica, Vol. 20, 2 (2014), pages 585–607.
We study the category of graded representations with finite-dimensional graded pieces for the current algebra associated to a simple Lie algebra. This category has many similarities with the category O of modules for g and in this paper, we use the combinatorics of Macdonald polynomials to prove an analogue of the famous BGG duality in the case of sln+1.
Primitively generated
Hall algebras (with J. Greenstein), Pacific Journal of Mathematics, Vol. 281, No. 2,
2016.
The aim of
the present paper is to demonstrate that Hall algebras of a large class of
finitary exact categories behave like quantum nilpotent groups in the sense
that they are generated by their primitive elements. Another goal is to
construct analogues of quantum enveloping algebras as certain primitively
generated subalgebras of the Hall algebras and conjecture an analogue of “Lie
correspondence” for those finitary categories.
Triangular bases in
quantum cluster algebras (with A.
Zelevinsky), Int. Math. Res. Not. 2012, no. 21, pages 4821–4883.
A lot of recent activity has been directed towards various
constructions of “natural” bases in cluster algebras. We develop a new approach
to this problem which is close in spirit to Lusztig’s construction of a
canonical basis, and the pioneering construction of the Kazhdan-Lusztig basis
in a Hecke algebra. The key ingredient of our approach is a new version of
Lusztig’s Lemma that we apply to all acyclic quantum cluster algebras. As a
result, we construct the “canonical” basis in every such algebra that we call
the canonical triangular basis.
The reciprocal of Σn≥0 anbn for non-commuting a and b,
Catalan
numbers and non-commutative quadratic equationsn for non-commuting a
and b, Catalan numbers and non-commutative quadratic equations (with V. Retakh, C.
Reutenauer, D.
Zeilberger ), Contemp.
Math. 592 (2013), 103–109.
The aim of this paper is to describe the inversion of the sum Σn≥0 anbn where a and b
are non-commuting variables as a formal series in a and b. We
show that the inversion satisfies a non-commutative quadratic equation and that
the number of certain monomials in its homogeneous components equals to a
Catalan number. We also study general solutions of similar quadratic equations.
Quantum Chevalley groups (with J. Greenstein), Contemp.
Math., 592 (2013), pages 71–102.
The goal of
this paper is to construct quantum analogues of Chevalley groups inside
completions of quantum groups or, more precisely, inside completions of Hall
algebras of finitary categories. In particular, we obtain pentagonal and other
identities in the quantum Chevalley groups which generalize their classical
counterparts and explain Faddeev-Volkov quantum dilogarithmic identities and
their recent generalizations due to Keller.
Littlewood-Richardson coefficients for
reflection groups (with E. Richmond), Advances in Mathematics, Vol 284 (2015), pages 54–111.
In this paper we
explicitly compute all Littlewood-Richardson coefficients for semisimple or
Kac-Moody groups G, that is, the structure coefficients of the
cohomology algebra H*(G/P), where P is a parabolic subgroup
of G. These coefficients are of importance in enumerative geometry,
algebraic combinatorics and representation theory. Our formula for the
Littlewood-Richardson coefficients is purely combinatorial and is given in
terms of the Cartan matrix and the Weyl group of G. In particular, our
formula gives a combinatorial proof of positivity of the Littlewood-Richardson
coefficients in the cases when off-diagonal Cartan matrix entries are less than
or equal to -2. Moreover, all our results for the Littlewood-Richardson
coefficients extend to the structure coefficients of the T-equivariant
cohomology algebra H*T(G/P).
A short proof of
Kontsevich cluster conjecture (with V.
Retakh),
C.
R. Math. Acad. Sci., Paris 349 (2011), no. 3–4, pages 119–122.
We give an
elementary proof of the Kontsevich conjecture that asserts that the iterations
of the noncommutative rational map Kr:(x,y)→(xyx-1,(1+y-r)x-1)
are given by noncommutative Laurent polynomials.
Stability inequalities
and universal Schubert calculus of rank 2 (with M. Kapovich), Transformation Groups, Vol. 16,
Issue 4 (2011), pages 955–1007.
The goal of
the paper is to introduce a version of Schubert calculus for each dihedral
reflection group W. That is, to each “sufficiently rich” spherical
building Y of type W we associate a certain cohomology theory and
verify that, first, it depends only on W (i.e., all such buildings are
“homotopy equivalent”) and second, the cohomology ring is the associated graded
of the coinvariant algebra of W under certain filtration. We also
construct the dual homology “pre-ring” of Y. The convex “stability”
cones defined via these (co)homology theories of Y are then shown to solve
the problem of classifying weighted semistable m-tuples on Y in
the sense of Kapovich, Leeb and Millson equivalently, they are cut out by the
generalized triangle inequalities for thick Euclidean buildings with the Tits
boundary Y. Quite remarkably, the cohomology ring is obtained from a
certain universal algebra A by a kind of “crystal limit” that has been
previously introduced by Belkale-Kumar for the cohomology of flag varieties and
Grassmannians. Another degeneration of A leads to the homology theory of
Y.
Quantum folding (with J. Greenstein), Int. Math. Res. Not. 2011, no. 21, pages
4821–4883.
In the present paper we
introduce a quantum analogue of the classical folding of a simply-laced Lie
algebra g to the non-simply-laced algebra gσ
along a Dynkin diagram automorphism σ of g. For each quantum
folding we replace gσ by its Langlands dual (gσ)v
and construct a nilpotent Lie algebra n which interpolates
between the nilpotent parts of g and (gσ)v,
together with its quantized enveloping algebra Uq(n) and
a Poisson structure on S(n). Remarkably, for the pair (g,
(gσ)v)=(so2n+2,sp2n),
the algebra Uq(n) admits an action of the Artin braid group Brn
and contains a new algebra of quantum n x n matrices with an adjoint
action of Uq(sln), which generalizes the algebras
constructed by K. Goodearl and M. Yakimov. The hardest case of quantum folding
is, quite expectably, the pair (so8,G2) for which the PBW
presentation of Uq(n) and the corresponding Poisson bracket on S(n)
contain more than 700 terms each.
Quasiharmonic
polynomials for Coxeter groups and representations of Cherednik algebras (with Yu.
Burman),
Trans.
Amer. Math. Soc., 362 (2010), 229–260.
We
introduce and study deformations of finite-dimensional modules over rational
Cherednik algebras. Our main tool is a generalization of usual harmonic
polynomials for Coxeter groups – the so-called quasiharmonic polynomials. A
surprising application of this approach is the construction of canonical
elementary symmetric polynomials and their deformations for all Coxeter groups.
Dunkl Operators and
Canonical Invariants of Reflection Groups (with Yu.
Burman),
SIGMA 5 (2009), 057, 18
pages.
Using Dunkl operators, we introduce a continuous family of
canonical invariants of finite reflection groups. We verify that the elementary
canonical invariants of the symmetric group are deformations of the elementary
symmetric polynomials. We also compute the canonical invariants for all
dihedral groups as certain hypergeometric functions.
Affine buildings for
dihedral groups (with M.
Kapovich),
Geometriae
Dedicata, 156 (2012), pages 171–207.
We
construct rank 2 thick nondiscrete affine buildings associated with an
arbitrary finite dihedral group.
Noncommutative Dunkl
operators and braided Cherednik algebras (with Y.
Bazlov)
Selecta
Mathematica, 14, (2009), pages 325–372.
We
introduce braided Dunkl operators that are acting on a q-polynomial
algebra and q-commute. Generalizing the approach of Etingof and
Ginzburg, we explain the q-commutation phenomenon by constructing
braided Cherednik algebras for which the above operators form a representation.
We classify all braided Cherednik algebras using the theory of braided doubles
developed in our previous paper. Besides ordinary rational Cherednik algebras,
our classification gives new algebras attached to an infinite family of subgroups
of even elements in complex reflection groups, so that the corresponding
braided Dunkl operators pairwise anti-commute. We explicitly compute these new
operators in terms of braided partial derivatives and divided differences.
Braided Doubles and
rational Cherednik algebras (with Y.
Bazlov),
Advances
in Mathematics, Vol. 220 (2009) 5, pages 1466–1530.
We
introduce and study a large class of algebras with triangular decomposition
which we call braided doubles. Braided doubles provide a unifying framework for
classical and quantum universal enveloping algebras and rational Cherednik
algebras. We classify braided doubles in terms of quasi-Yetter-Drinfeld (QYD)
modules over Hopf algebras which turn out to be a generalisation of the
ordinary Yetter-Drinfeld modules. To each braiding (a solution to the braid
equation) we associate a QYD-module and the corresponding braided Heisenberg
double — this is a quantum deformation of the Weyl algebra where the role of
polynomial algebras is played by Nichols-Woronowicz algebras. Our main result
is that any rational Cherednik algebra canonically embeds into the braided Heisenberg
double attached to the corresponding complex reflection group.
Lie algebras and Lie
groups over noncommutative rings (with V.
Retakh),
Advances
in Mathematics, Vol. 218, 6, (2008), pages 1723–1758.
The aim of
this paper is to introduce and study Lie algebras over noncommutative rings.
For any Lie algebra g sitting inside an associative algebra A and
any associative algebra F we introduce and study the F-current
Lie algebra (g,A)(F), which is the Lie subalgebra of F⊗A generated by F⊗g. In most examples A
is the universal enveloping algebra of g. Our description of the
current algebra has a striking resemblance to the commutator expansions of F
used by M. Kapranov in his approach to noncommutative geometry. We also
associate with each Lie algebra (g,A)(F) a “noncommutative
algebraic” group G which naturally acts on (g,A)(F) by
conjugations and conclude the paper with a number of examples of such groups.
Braided symmetric and
exterior algebras (with S.
Zwicknagl), Trans.
Amer. Math. Soc., 360 (2008), pages 3429–3472.
We
introduce and study symmetric and exterior algebras in braided monoidal
categories such as the category O over quantum groups. We relate our
braided symmetric algebras and braided exterior algebras with their classical
counterparts.
Lecture notes on
geometric crystals and their combinatorial analogues (with D. Kazhdan), Combinatorial aspect of integrable
systems, MSJ
Memoirs, 17, Mathematical Society of Japan, Tokyo, 2007.
This is an
exposition of the results on Geometric crystals and the associated Kashiwara
crystal bases (presented by the first author in RIMS, August 2004).
Geometric and Unipotent
Crystals II: From Unipotent Bicrystals to Crystal Bases (with D. Kazhdan), Contemp.
Math., 433, Amer. Math. Soc., Providence, RI, 2007, pages 13–88.
For each
reductive algebraic group G, we introduce and study unipotent
bicrystals which serve as a regular version of rational geometric and
unipotent crystals introduced earlier by the authors. The framework of
unipotent bicrystals allows, on the one hand, to study systematically such
varieties as Bruhat cells in G and their convolution products and, on
the other hand, to give a new construction of many normal Kashiwara crystals
including those for Gv-modules, where Gv is
the Langlands dual groups. In fact, our analogues of crystal bases (which
we refer to as crystals associated to Gv-modules) are
associated to Gv-modules directly, i.e., without quantum
deformations.
Noncommutative Double Bruhat cells and their factorizations (with V. Retakh), Int. Math. Res. Not., 8 (2005), pages 477–516.
In the present paper we study noncommutative double Bruhat cells. Our main results are explicit positive matrix factorizations in the cells via quasiminors of matrices with noncommutative coefficients.
Quantum cluster algebras (with A. Zelevinsky) Advances in Mathematics, vol. 195, 2 (2005), pages 405–455.
Cluster algebras were introduced by S. Fomin and A. Zelevinsky; their study continued in a series of papers including Cluster algebras III: Upper bounds and double Bruhat cells. This is a family of commutative rings designed to serve as an algebraic framework for the theory of total positivity and canonical bases in semisimple groups and their quantum analogs. In this paper we introduce and study quantum deformations of cluster algebras.
Cluster algebras III: Upper bounds and double Bruhat cells (with S. Fomin and A. Zelevinsky) Duke Math. Journal, vol. 126, 1 (2005), pages 1–52.
We continue the study of cluster algebras. We develop a new approach based on the notion of upper cluster algebra, defined as an intersection of certain Laurent polynomial rings. Strengthening the Laurent phenomenon, we show that, under an assumption of “acyclicity,” a cluster algebra coincides with its “upper” counterpart, and is finitely generated. In this case, we also describe its defining ideal, and construct a standard monomial basis. We prove that the coordinate ring of any double Bruhat cell in a semisimple complex Lie group is naturally isomorphic to the upper cluster algebra explicitly defined in terms of relevant combinatorial data.
Tensor product multiplicities, canonical bases and totally positive varieties (with A. Zelevinsky) Invent. Math., vol. 143, 1 (2001), pages 77–128.
We obtain a family of explicit “polyhedral” combinatorial expressions for multiplicities in the tensor product of two simple finite-dimensional modules over a complex semisimple Lie algebra. Here “polyhedral” means that the multiplicity in question is expressed as the number of lattice points in some convex polytope. Our answers use a new combinatorial concept of i-trails which resemble Littelmann’s paths but seem to be more tractable. We also study combinatorial structure of Lusztig’s canonical bases or, equivalently of Kashiwara’s global bases. Although Lusztig’s and Kashiwara’s approaches were shown by Lusztig to be equivalent to each other, they lead to different combinatorial parametrizations of the canonical bases. One of our main results is an explicit description of the relationship between these parametrizations. Our approach to the above problems is based on a remarkable observation by G. Lusztig that combinatorics of the canonical basis is closely related to geometry of the totally positive varieties. We formulate this relationship in terms of two mutually inverse transformations: “tropicalization” and “geometric lifting.”
Geometric and unipotent crystals (with D. Kazhdan) Geom.
Funct. Anal., Special Volume, Part I (2000), pages 188–236.
We
introduce geometric crystals and unipotent crystals which are algebro-geometric
analogues of Kashiwara’s crystal bases. Given a reductive group G, let I
be the set of vertices of the Dynkin diagram of G and T be the
maximal torus of G. The structure of a geometric G-crystal on
an algebraic variety X consists of a rational morphism γ:XàT
and a compatible family ei:Gm×XàX,
i in I of rational actions of the multiplicative
group Gm satisfying certain braid-like
relations. Such a structure induces a rational action of W on X.
Surprisingly many interesting rational actions of the group W come
from geometric crystals. Also all the known examples of the action of W
which appear in the construction of Gamma-functions for the
representations of the Langlands dual group Gv in the
recent work by A. Braverman and D. Kazhdan come from geometric
crystals. There are many examples of positive geometric crystals on (Gm)l,
i.e., those geometric crystals for which the actions ei and
the morphism gamma are given by positive rational expressions. One
can associate to each positive geometric crystal X the Kashiwara’s
crystal corresponding to the Langlands dual group Gv.
An emergence of Gv in the “crystal world” was observed
earlier by G. Lusztig. Another application of geometric crystals is a
construction of trivialization which is an W-equivariant isomorphism
Xà>γ-1(e)×T for any geometric SLn-crystal.
Unipotent crystals are geometric analogues of normal Kashiwara crystals. They
form a strict monoidal category. To any unipotent crystal built on a variety X
we associate a certain geometric crystal.
Coadjoint orbits, moment polytopes, and the
Hilbert-Mumford criterion
(with R.
Sjamaar), J. Amer. Math. Soc., 13 (2000), no. 2, pages 433–466.
In this paper we solve
of the following problem: Given a reductive group G, and its reductive
subgroup H, describe the momentum cone Δo. This is a rational polyhedral
cone spanned by all those dominant G-weights λ for which the
simple G-module Vλ contains a non-trivial H-invariant.
Our result generalizes the result by Klyachko who has solved this problem for G=
GLn×GLn×GLn with the subgroup H=GLn embedded diagonally into G.
We describe the facets of the cone Δo in terms of the “relative”
Schubert calculus of the flag varieties of the two groups. Another formulation
of the result is the description of the relative momentum cone Δ, which
is spanned by those pairs (λ,λ') for which the
restriction to H of the simple G-module Vλ
contains a simple H-module V'λ'.
Domino tableaux, Schutzenberger involution and action of the symmetric group (with Anatol Kirillov), Proceedings of the 10th International Conference on Formal Power Series and Algebraic Combinatorics, Fields Institute, Toronto, 1998, Discrete Math., vol. 225, 1–3 (2000), pages 5–24.
Concavity of weighted arithmetic means with applications (with Alek Vainshtein), Arch. Math. (1997) 69, pages 120–126.
Total positivity in Schubert varieties (with A. Zelevinsky) Comment. Math. Helv. 72 (1997), no. 1, pages 128–166.
In this paper we further develop the remarkable parallelism discovered by Lusztig between the canonical basis and the variety of totally positive elements in the unipotent group.
Parametrizations of canonical bases and totally positive matrices (with S. Fomin and A. Zelevinsky), Advances in Mathematics 122 (1996), pages 49–149.
We provide: (i) explicit formulas for Lusztig’s transition maps related to the canonical basis of the quantum group of type A; (ii) formulas for the factorizations of a square matrix into elementary Jacobi matrices; (iii) a family of new total positivity criteria.
Group-like elements in quantum groups and Feigin’s conjecture, preprint.
In this paper analogue of the Gelfand-Kirillov conjecture for any simple quantum group Gq is proved (here Gq is the q-deformed coordinate ring of a simple algebraic group G). Namely, the field of fractions of Gq is isomorphic to the field of fractions of a certain skew-polynomial ring. The proof is based on a construction of some group-like elements in Gq (which are q-analogs of elements in G).
Canonical bases for the quantum group of type Ar and piecewise-linear
combinatorics
(with A.
Zelevinsky), Duke
Math. J. 82 (1996), no. 3, pages 473–502.
We use the structure
theory of the dual canonical basis B is to obtain a direct
representation-theoretic proof of the Littlewood-Richardson rule (or rather,
its piecewise-linear versions discussed above). Another application of string
technique is an explicit formula for the action of the longest element wo in Sr+1 on the dual canonical basis in
each simple slr+1-module. Having been translated
into the language of Gelfand-Tsetlin patterns and Young tableaux, this
involution coincides with the Schützenberger involution.
String bases for quantum groups of type Ar (with A. Zelevinsky) I. M. Gelfand Seminar, 51–89, Adv. Soviet Math., 16, Part 1, Amer. Math. Soc., Providence, RI, 1993.
We introduce and study a family of string bases for the quantum groups of type Ar (which includes the dual canonical basis). These bases are defined axiomatically and possess many interesting properties, e.g., they all are good in the sense of Gelfand and Zelevinsky. For every string basis, we construct a family of combinatorial labelings by strings. These labelings in a different context appeared in more recent works by M. Kashiwara and by P. Littelmann. We expect that B has a nice multiplicative structure. Namely, we conjecture in [8] that B contains all products of pairwise q-commuting elements of B. The conjecture was proved in [8] for A2 and A3. In fact, for r< 4, the dual canonical basis B is the only string basis and it consists of all q-commuting products of quantum minors (for r arbitrary, we proved that any string basis contains all quantum minors).
Groups generated by involutions, Gel’fand-Tsetlin patterns, and combinatorics of Young tableaux (with Anatol Kirillov), Algebra i Analiz 7 (1995), no. 1, 92–152 (Russian). Translation in: St. Petersburg Math. J., 7 (1996), no. 1, pages 77–127.
The original motivation of this paper was to understand a rather mysterious action of the symmetric group Sn on Young tableaux, discovered by Lascoux and Schutzenberger. We introduced an action of Sn by piecewise-linear transformations on the space of Gelfand-Tsetlin patterns. In our approach, this group appears as a subgroup of the infinite group Gn, generated by quite simple piecewise-linear involutions (these involutions are continuous analogues of Bender-Knuth involutions acting on Young tableaux). The structure of Gn is not yet completely understood. Some relations were given in [7]; they involve the famous Schützenberger involution which also belongs to Gn. Another result of [7] is a conjectural description of Kashiwara’s crystal operators for type A, in terms of Gn.
Triple multiplicities for sl(r+1) and the spectrum of the exterior algebra of the adjoint representation (with A. Zelevinsky), J. Algebraic Combin. 1 (1992), no. 1, pages 7–22.
When is the weight multiplicity equal to 1 (Russian) (with A. Zelevinsky) Funkc. Anal. Pril. 24 (1990), no. 4, 1–13; translation: Funct. Anal. Appl. 24 (1990), no. 4, pages 259–269.
Tensor product multiplicities and convex polytopes in partition space (with A. Zelevinsky) J. Geom. Phys. 5 (1988), no. 3, pages 453–472.
A multiplicative analogue of the Bergstrom inequality for a matrix product in the sense of Hadamard (with Alek Vainshtein) (Russian) Uspekhi Mat. Nauk 42 (1987), no. 6 (258), pages 181–182. Translation: Russian Mathematical Surveys.
The convexity property of the Poisson distribution and its applications in queueing theory (with Alek Vainshtein and A. Kreinin) (Russian). Translation: J. Soviet Math. 47 (1989), no. 1.
Involutions on Gelfand-Tsetlin patterns and multiplicities in skew GL(n)-modules (with A. Zelevinsky) Soviet Math. Dokl. 37 (1988), no. 3, 799–802 592 (2013), pages 71–102.