Low-dimensional representations of Aut (F 2)

LetF ₂ be the free group of rank two, and Φ₂ its automorphism group. We consider the problem of describing the representations of Φ₂ of degreen for small values ofn. Our main result is the classification (up to equivalence) of all indecomposable representations ρ of Φ₂ of degreen≤4 such that ρ(F ₂) ≠ 1. There are only finitely many such representations, and in all them ρ(F ₂) is solvable. This is no longer true in higher dimensions. Already forn=6 there exists a 1-parameter family of irreducible nonequivalent representations of Φ₂ such that ρ(F ₂) contains a free non-abelian subgroup. We also obtain some new 4-dimensional representations of the braid groupB ₄ which are indecomposable and reducible at the same time. It would be interesting to find some applications of these representations. Supported in part by the NSERC Grant A-5285 Supported in part by an NSERC grant     

Irreducibility of some Faber polynomials

Apply weight 0 Hecke operators to the modular function j and express the result as a polynomial in j. These polynomials were considered long ago in analysis, and recently attracted the attention of number theorists primarily for their connection with Borcherds’ infinite products. In particular, Ken Ono conjectured that all of them are irreducible. We prove a partial result towards this conjecture by presenting infinite families of these polynomials which are proved to be irreducible. Supported by NSF grant DMS-0501225.     

Relative Haag Duality for the Free Field in Fock Representation

We consider a natural generalization of Haag duality to the case in which the observable algebra is restricted to a subset of the space-time and is not irreducible: the commutant and the causal complement have to be considered relatively to the ambient space. We prove this relative form of Haag duality under quite general conditions for the free scalar and electromagnetic field of space dimension d ≥ 2 in the vacuum representation. Such property is interesting in view of a theory of superselection sectors for the electromagnetic field. Supported by the EU network “Quantum Spaces – Noncommutative Geometry” HPRN-CT-2002-00280. Submitted: August 1, 2006. Accepted: March 2, 2007.     

Conformal nets associated with lattices and their orbifolds

In this paper, we study representations of conformal nets associated with positive definite even lattices and their orbifolds with respect to isometries of the lattices. Using previous general results on orbifolds, we give a list of all irreducible representations of the orbifolds, which generate a unitary modular tensor category.     

A note on trilinear forms for reducible representations and Beilinson’s conjectures

We extend Prasad’s results on the existence of trilinear forms on representations of GL ₂ of a local field, by permitting one or more of the representations to be reducible principal series, with infinite-dimensional irreducible quotient. We apply this in a global setting to compute (unconditionally) the dimensions of the subspaces of motivic cohomology of the product of two modular curves constructed by Beilinson. Received February 24, 2000 / final version received September 12, 2000?Published online November 8, 2000     

Regular unipotent elements from subsystem subgroups of rank 2 in modular representations of classical groups

Images of regular unipotent elements from subsystem subgroups of type A₂ and B₂ in irreducible modular representations of classical groups are studied. For images of such elements and representations with locally small highest weights, all sizes of Jordan blocks of one and the same parity are found. Bibliography: 17 titles. Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 356, 2008, pp. 159–178.     

Parabolic bundles and representations¶of the fundamental group

Let X be a smooth complex projective variety with Neron–Severi group isomorphic to ℤ, and D an irreducible divisor with normal crossing singularities. Assume 1<r≤ 3. We prove that if π₁(X) doesn't have irreducible PU(r) representations, then π₁(X- D) doesn't have irreducible U(r) representations. The proof uses the non-existence of certain stable parabolic bundles. We also obtain a similar result for GL(2) when D is smooth. Received: 20 December 1999 / Revised version: 7 May 2000     

Equivalence of commutation relations

We consider C*-algebras of commutation relations over the fields ℚ _p, p = 2, 3, 5, …, ∞. We describe all the irreducible separable representations of these algebras. We prove that the algebras are not isomorphic at different p. __________ Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 157, No. 3, pp. 406–412, December, 2008.     

Modular irreducible representations of the symmetric group as linear codes

We describe a particularly easy way of evaluating the modular irreducible matrix representations of the symmetric group. It shows that Specht’s approach to the ordinary irreducible representations, along Specht polynomials, can be unified with Clausen’s approach to the modular irreducible representations using symmetrized standard bideterminants. The unified method, using symmetrized Specht polynomials, is very easy to explain, and it follows directly from Clausen’s theorem by replacing the indeterminate x_ij of the letter place algebra by x_jⁱ.Our approach is implemented in SYMMETRICA. It was used in order to obtain computational results on code theoretic properties of the p-modular irreducible representation [λ]_p corresponding to a p-regular partition λ via embedding it into representation spaces obtained from ordinary irreducible representations. The first embedding is into the permutation representation induced from the column group of a standard Young tableau of shape λ. The second embedding is the embedding of [λ]_p into the space of , the p-modular representation obtained from the ordinary irreducible representation [λ] by reducing the coefficients modulo p.We include a few tables with dimensions and minimum distances of these codes; others can be found via our home page.     

On the representation theory of an algebra of braids and ties

We consider the algebra ℰ _n (u) introduced by Aicardi and Juyumaya as an abstraction of the Yokonuma–Hecke algebra. We construct a tensor space representation for ℰ _n (u) and show that this is faithful. We use it to give a basis of ℰ _n (u) and to classify its irreducible representations.     

Left-symmetric superalgebra structures on the N = 2 superconformal algebras

The super-Virasoro algebras, also known as the superconformal algebras, are nontrivial graded extensions of the Virasoro algebra to Lie superalgebra version. In this paper, we classify the compatible left-symmetric superalgebra structures on the N = 2 Ramond and Neveu–Schwarz superconformal algebras under certain conditions, which generalizes the corresponding results for the Witt, Virasoro and super Virasoro algebras.     

Unitarizability of a certain class of irreducible representations of classical groups

We prove that a certain class of irreducible representations of the classical p-adic groups is unitarizable and in general, can be isolated in the unitary dual. These representations are Aubert duals of a certain class of square-integrable representations, thus, in this case, Bernstein’s conjecture, which states that the Aubert involution preserves unitarizability, is confirmed.     

Explicit irreducible representations of the¶Iwahori–Hecke algebra of Type F4

A general method for computing irreducible representations of Weyl groups and Iwahori–Hecke algebras was introduced by the first author in [10]. In that paper the representations of the algebras of types A _n , B _n , D _n and G _n were computed and it is the purpose of this paper to extend these computations to F ₄. The main goal here is to compute irreducible representations of the Iwahori–Hecke algebra of type F ₄ by only using information in the character table of the Weyl group. Received: Received: 30 July 1998     

Free group representations from vector-valued multiplicative functions,I

Let Γ denote a noncommutative free group and let Ω stand for its boundary. We construct a large class of unitary representations of Γ. This class contains many previously studied representations, and is closed under several natural operations. Each of the constructed representations is in fact a representation of Γ ⋉_λ C(Ω). We prove here that each of them is irreducible as a representation of Γ ⋉_λ C(Ω). Actually, as will be shown in further work, each of them is irreducible as a representation of Γ, or is the direct sum of exactly two irreducible, inequivalent Γ-representations. This research was supported by the Italian CNR.     

Effective transverse elastic moduli of composites at non-dilute concentration of a random field of aligned fibers

We consider a transversal loading of a linearly elastic isotropic media containing the identical isotropic aligned circular fibers at non-dilute concentration c. By the use of solution obtained by the Kolosov–Muskhelishvili complex potential method for two interacting circles subjected to three different applied stresses at infinity, and exact integral representations for both the stress and strain distributions in a microinhomogeneous medium, one estimates the effective moduli of the composite accurately to order c². Received: March 4, 2003; revised: August 8, 2003     

Local conformal nets arising from framed vertex operator algebras

We apply an idea of framed vertex operator algebras to a construction of local conformal nets of (injective type III₁) factors on the circle corresponding to various lattice vertex operator algebras and their twisted orbifolds. In particular, we give a local conformal net corresponding to the moonshine vertex operator algebras of Frenkel-Lepowsky-Meurman. Its central charge is 24, it has a trivial representation theory in the sense that the vacuum sector is the only irreducible DHR sector, its vacuum character is the modular invariant J-function and its automorphism group (the gauge group) is the Monster group. We use our previous tools such as α-induction and complete rationality to study extensions of local conformal nets.     

A new correctness criterion for multiplicative non-commutative proof nets

 We introduce a new correctness criterion for multiplicative non commutative proof nets which can be considered as the non-commutative counterpart to the Danos-Regnier criterion for proof nets of linear logic. The main intuition relies on the fact that any switching for a proof net (obtained by mutilating one premise of each disjunction link) can be naturally viewed as a series-parallel order variety (a cyclic relation) on the conclusions of the proof net. Received: 8 November 2000 / Revised version: 21 June 2001 / Published online: 2 September 2002 Research supported by the EU TMR Research Programme ``Linear Logic and Theoretical Computer Science'. Mathematics Subject Classification (2000): 03F03, 03F07, 03F52, 03B70 Key words or phrases: Linear and non-commutative logic – Proof nets – Series-parallel orders and order varieties     

The Six-Vertex Model at Roots of Unity and some Highest Weight Representations of the sl 2 Loop Algebra

We discuss irreducible highest weight representations of the sl₂ loop algebra and reducible indecomposable ones in association with the sl₂ loop algebra symmetry of the six-vertex model at roots of unity. We formulate an elementary proof that every highest weight representation with distinct evaluation parameters is irreducible. We present a general criteria for a highest weight representation to be irreducible. We also give an example of a reducible indecomposable highest weight representation and discuss its dimensionality. Communicated by Vincent Rivasseau Dedicated to Daniel Arnaudon Submitted: March 3, 2006; Accepted: March 13, 2006     

A geometric approach to the Kronecker problem I: The two row case

Given two irreducible representations μ, v of the symmetric group S _d , the Kronecker problem is to find an explicit rule, giving the multiplicity of an irreducible representation, λ, of S _d , in the tensor product of μ and v. We propose a geometric approach to investigate this problem. We demonstrate its effectiveness by obtaining explicit formulas for the tensor product multiplicities, when the irreducible representations are parameterized by partitions with at most two rows.