Root polytopes and Abelian ideals

We study the root polytope $\mathcal{P}_{\varPhi}$ of a finite irreducible crystallographic root system Φ using its relation with the Abelian ideals of a Borel subalgebra of a simple Lie algebra with root system Φ. We determine the hyperplane arrangement corresponding to the faces of codimension 2 of $\mathcal{P}_{\varPhi}$ and analyze its relation with the facets of $\mathcal{P}_{\varPhi}$ . For Φ of type A _n or C _n , we show that the orbits of some special subsets of Abelian ideals under the action of the Weyl group parametrize a triangulation of  $\mathcal{P}_{\varPhi}$ . We show that this triangulation restricts to a triangulation of the positive root polytope  $\mathcal{P}_{\varPhi}^{+}$ .     

Convergence and relative compactness in distribution of sequences of random hypermeasures

Let Φ be a compact set in a vector space equipped with a convergence which is metrizable in Φ but not certainly in the whole space. We endow the space of continuous on Φ linear functionals on span Φ with the norm \( {\left\| u \right\|_\Phi } = \sup \varphi \in \Phi \left| {u\varphi } \right| \) and call the elements of the completion of Φ hypermeasures. We prove theorems on the convergence in probability or in distribution and relative compactness in distribution of a sequence of random hypermeasures.     

Admissible wavelets associated with the classical domain of type one

The classical domain of type one has an unbounded realization as the Siegeldomain D(Φ,Ω)by the Cayley transform.Let P be the Iwasawa subgroupof the affine automorphisms group of D(Φ,Ω),then P has a natural uni-tary representation U on L~2.We decompose L~2into the direct sumof the irreducible invariant closed subspaces under U,and give the char-acterization of the admissible condition in terms of the Fourier transform.Define the wavelet transform,we obtain the direct sum.decomposition ofL~2(D(Φ,Ω),dμ).     

On self-similar solutions of semilinear wave equations in higher space dimensions

In this paper we analyze self-similar solutions of the semilinear wave equation Φ_tt − ΔΦ − Φ^p = 0 for n > 3 space dimensions. We found several classes of analytic solutions labeled by a single parameter, the form of which differ in the vicinity of the light cone. We also propose suitable numerical methods to study them.     

Application of Vertex Algebras to the Structure Theory of Certain Representations Over the Virasoro Algebra

In this paper, we discuss the structure of the tensor product \(V_{\alpha,\beta }^{\prime}\otimes L(c,h)\) of an irreducible module from an intermediate series and irreducible highest-weight module over the Virasoro algebra. We generalize Zhang’s irreducibility criterion from Zhang (J Algebra 190:1–10, 1997), and show that irreducibility depends on the existence of integral roots of a certain polynomial, induced by a singular vector in the Verma module V(c,h). A new type of irreducible Vir-module with infinite-dimensional weight subspaces is found. We show how the existence of intertwining operators for modules over vertex operator algebra yields reducibility of \(V_{\alpha ,\beta}^{\prime}\otimes L(c,h)\) , which is a completely new point of view to this problem. As an example, the complete structure of the tensor product with minimal models c?=???22/5 and c?=?1/2 is presented.     

Embeddings between weak Orlicz martingale spaces

Let Φ be an increasing and convex function on [0,∞) with Φ(0)=0 satisfying that for any α>0, there exists a positive constant Cα such that Φ(αt)?CαΦ(t), t>0. Let wLΦ denote the corresponding weak Orlicz space. We obtain some embeddings between vector-valued weak Orlicz martingale spaces by establishing the wLΦ-inequalities for martingale transform operators with operator-valued multiplying sequences. These embeddings are closely related to the geometric properties of the underlying Banach space. In particular, for any scalar valued martingale f=(fn)_n?1, we claim that

     

Irreducible pseudo 2-factor isomorphic cubic bipartite graphs

A bipartite graph is pseudo 2-factor isomorphic if the number of circuits in each 2-factor of the graph is always even or always odd. We proved (Abreu et?al., J Comb Theory B 98:432–442, 2008) that the only essentially 4-edge-connected pseudo 2-factor isomorphic cubic bipartite graph of girth 4 is K _3,3, and conjectured (Abreu et?al., 2008, Conjecture 3.6) that the only essentially 4-edge-connected cubic bipartite graphs are K _3,3, the Heawood graph and the Pappus graph. There exists a characterization of symmetric configurations n ₃ due to Martinetti (1886) in which all symmetric configurations n ₃ can be obtained from an infinite set of so called irreducible configurations (Martinetti, Annali di Matematica Pura ed Applicata II 15:1–26, 1888). The list of irreducible configurations has been completed by Boben (Discret Math 307:331–344, 2007) in terms of their irreducible Levi graphs. In this paper we characterize irreducible pseudo 2-factor isomorphic cubic bipartite graphs proving that the only pseudo 2-factor isomorphic irreducible Levi graphs are the Heawood and Pappus graphs. Moreover, the obtained characterization allows us to partially prove the above Conjecture.     

Séparation des orbites coadjointes d'un groupe exponentiel par leur enveloppe convexe

Looking to the separation of irreducible unitary representations of an exponential Lie group G through the image of their moment map, we propose here a new way: instead to extend the moment map to the universal enveloping algebra of G, we define a non linear mapping Φ from the dual of the Lie algebra g of G to the dual g^+∗ of a larger solvable group G⁺, and we extend the representation from G to G⁺, in such a manner that the corresponding coadjoint orbits in g^+∗ have distinct closed convex hull. This allows us to separate the irreducible unitary representations of G.     

A characterization of Leonard pairs using the notion of a tail

Let V denote a vector space with finite positive dimension. We consider an ordered pair of linear transformations A:V→V and A^∗:V→V that satisfy (i) and (ii) below:

(i)

There exists a basis for V with respect to which the matrix representing A is irreducible tridiagonal and the matrix representing A^∗ is diagonal.

(ii)

There exists a basis for V with respect to which the matrix representing A^∗ is irreducible tridiagonal and the matrix representing A is diagonal.

We call such a pair a Leonard pair on V. In this paper, we characterize the Leonard pairs using the notion of a tail. This notion is borrowed from algebraic graph theory.     

Toroidal and Klein bottle boundary slopes

Let M be a compact, connected, orientable, irreducible 3-manifold and T₀ an incompressible torus boundary component of M such that the pair (M,T₀) is not cabled. By a result of C. Gordon, if (S,∂S),(T,∂T)⊂(M,T₀) are incompressible punctured tori with boundary slopes at distance Δ=Δ(∂S,∂T), then Δ?8, and the cases where Δ=6,7,8 are very few and classified. We give a simplified proof of this result (or rather, of its reduction process), using an improved estimate for the maximum possible number of mutually parallel negative edges in the graphs of intersection of S and T. We also extend Gordon's result by allowing either S or T to be an essential Klein bottle.     

Reducibility of matrix weights

In this paper, we discuss the notion of reducibility of matrix weights and introduce a real vector space \(\mathcal C_\mathbb R\) which encodes all information about the reducibility of W. In particular, a weight W reduces if and only if there is a nonscalar matrix T such that \(TW=WT^*\). Also, we prove that reducibility can be studied by looking at the commutant of the monic orthogonal polynomials or by looking at the coefficients of the corresponding three-term recursion relation. A matrix weight may not be expressible as direct sum of irreducible weights, but it is always equivalent to a direct sum of irreducible weights. We also establish that the decompositions of two equivalent weights as sums of irreducible weights have the same number of terms and that, up to a permutation, they are equivalent. We consider the algebra of right-hand-side matrix differential operators \(\mathcal D(W)\) of a reducible weight W, giving its general structure. Finally, we make a change of emphasis by considering the reducibility of polynomials, instead of reducibility of matrix weights.     

Block characters of the symmetric groups

A block character of a finite symmetric group is a positive definite function which depends only on the number of cycles in a permutation. We describe the cone of block characters by identifying its extreme rays, and find relations of the characters to descent representations and the coinvariant algebra of ${\mathfrak{S}}_{n}$ . The decomposition of extreme block characters into the sum of characters of irreducible representations gives rise to certain limit shape theorems for random Young diagrams. We also study counterparts of the block characters for the infinite symmetric group ${\mathfrak{S}}_{\infty}$ , along with their connection to the Thoma characters of the infinite linear group GL _∞(q) over a Galois field.     

On approximation by polynomials and rational functions in Orlicz spaces

Для пространств Орли ча получен аналог изв естного неравенства С. Б. Стечк ина об оценке наименьших по линомиальных уклоне ний через модуль гладкости про извольного порядка. Например, если?∈L* _Φ (I), то \(R_n (f,I)_\Phi \leqq E(f,I)_\Phi \leqq C(\Phi ,r)\omega _r \left( {\tfrac{1}{n},f,I} \right)_\Phi \) при всех натуральныхr иn≧r (теорема I). Доказана неулучшаем ость этой теоремы, ее а налог для случая приближения т ригонометрическими полиномами и тригоно метрическими рацион альными функциями. Установлена связьΔ ₂-условия на функциюΦ(u) со свойствами аппрокси мации соответствующ их классов функций (теор ема 3).     

Linear colorings of simplicial complexes and collapsing

A vertex coloring of a simplicial complex Δ is called a linear coloring if it satisfies the property that for every pair of facets (F₁,F₂) of Δ, there exists no pair of vertices (v₁,v₂) with the same color such that v₁∈F₁?F₂ and v₂∈F₂?F₁. The linear chromatic numberlchr(Δ) of Δ is defined as the minimum integer k such that Δ has a linear coloring with k colors. We show that if Δ is a simplicial complex with lchr(Δ)=k, then it has a subcomplex Δ^′ with k vertices such that Δ is simple homotopy equivalent to Δ^′. As a corollary, we obtain that lchr(Δ)?Homdim(Δ)+2. We also show in the case of linearly colored simplicial complexes, the usual assignment of a simplicial complex to a multicomplex has an inverse. Finally, we show that the chromatic number of a simple graph is bounded from above by the linear chromatic number of its neighborhood complex.     

On the Number of Irreducible Points in Polyhedra

An integer point in a polyhedron is called irreducible iff it is not the midpoint of two other integer points in the polyhedron. We prove that the number of irreducible integer points in n-dimensional polytope P is at most \(O(m^{\lfloor \frac{n}{2}\rfloor }\log ^{n-1}\gamma )\), where n is fixed and P is given by a system of m linear inequalities with integer coefficients not exceeding (by absolute value) \(\gamma \). This bound is tight. Using this result we prove the conjecture asserting that the teaching dimension in the class of threshold functions of k-valued logic in n variables is \(\varTheta (\log ^{n-2} k)\) for any fixed \(n\ge 2\).     

A conjugated version of the countable dense homogeneity

We are interested in finding a homeomorphism h of a space X with h⁻¹○Φ○h(A)=B for a given bijection Φ of X and every pair of countable dense subsets A and B of X. For a separable Banach space X, such a homeomorphism h always exists provided the fixed-point set of Φ has the empty interior. Moreover, h can be chosen to be real-analytic. As a consequence, there exists a real analytic flow that sends A onto B after time t=1. Actually, for X=Rn, any bounded real-analytic vector field can be approximated by a real-analytic vector field whose induced flow sends A onto B after time t=1. Topological and Cp smooth counterparts of these results are also obtained.     

Derivation radical subspace arrangements

In this note we study modules of derivations on collections of linear subspaces in a finite dimensional vector space. The central aim is to generalize the notion of freeness from hyperplane arrangements to subspace arrangements. We call this generalization ‘derivation radical’. We classify all coordinate subspace arrangements that are derivation radical and show that certain subspace arrangements of the Braid arrangement are derivation radical. We conclude by proving that under an algebraic condition the subspace arrangement consisting of all codimension c intersections, where c is fixed, of a free hyperplane arrangement are derivation radical.     

Hoffman polynomials of nonnegative irreducible matrices and strongly connected digraphs

For a nonnegative n × n matrix A, we find that there is a polynomial f(x)∈R[x] such that f(A) is a positive matrix of rank one if and only if A is irreducible. Furthermore, we show that the lowest degree such polynomial f(x) with tr f(A) = n is unique. Thus, generalizing the well-known definition of the Hoffman polynomial of a strongly connected regular digraph, for any irreducible nonnegative n × n matrix A, we are led to define its Hoffman polynomial to be the polynomial f(x) of minimum degree satisfying that f(A) is positive and has rank 1 and trace n. The Hoffman polynomial of a strongly connected digraph is defined to be the Hoffman polynomial of its adjacency matrix. We collect in this paper some basic results and open problems related to the concept of Hoffman polynomials.