On the Automorphism Group of a Smooth Schubert Variety

Let G be a simple algebraic group of adjoint type over the field \(\mathbb {C}\) of complex numbers. Let B be a Borel subgroup of G containing a maximal torus T of G. Let w be an element of the Weyl group W and let X(w) be the Schubert variety in G/B corresponding to w. Let α ₀ denote the highest root of G with respect to T and B. Let P be the stabiliser of X(w) in G. In this paper, we prove that if G is simply laced and X(w) is smooth, then the connected component of the automorphism group of X(w) containing the identity automorphism equals P if and only if w ^?1(α ₀) is a negative root (see Theorem 4.2). We prove a partial result in the non simply laced case (see Theorem 6.6).     

Word-Representable Graphs: a Survey

Letters x and y alternate in a word w if after deleting all letters but x and y in w we get either a word xyxy... or a word yxyx... (each of these words can be of odd or even length). A graph G = (V,E) is word-representable if there is a finite word w over an alphabet V such that the letters x and y alternate in w if and only if xy ∈ E. The word-representable graphs include many important graph classes, in particular, circle graphs, 3-colorable graphs and comparability graphs. In this paper we present the full survey of the available results on the theory of word-representable graphs and the most recent achievements in this field.     

On <Emphasis Type="BoldItalic">S</Emphasis>-strong Mori modules

Let A be an integral domain, \(S\subseteq A\) be a multiplicative set and M a w-module as an A-module. In this paper we investigate S-SM-modules. We give an S-version of the result of Wang and McCasland (Commun Algebra 25:1285–1306, 1997) in the case where S is countable. We prove that M is an S-SM-module if and only if every increasing sequence of w-submodules of M is S-stationary if and only if every increasing sequence of S-w-finite w-submodules of M is S-stationary if and only if every increasing sequence of w-finite type submodules of M is S-stationary.     

Word-Representability of Face Subdivisions of Triangular Grid Graphs

A graph \(G=(V,E)\) is word-representable if there exists a word w over the alphabet V such that letters x and y alternate in w if and only if \((x,y)\in E\). A triangular grid graph is a subgraph of a tiling of the plane with equilateral triangles defined by a finite number of triangles, called cells. A face subdivision of a triangular grid graph is replacing some of its cells by plane copies of the complete graph \(K_4\). Inspired by a recent elegant result of Akrobotu et al., who classified word-representable triangulations of grid graphs related to convex polyominoes, we characterize word-representable face subdivisions of triangular grid graphs. A key role in the characterization is played by smart orientations introduced by us in this paper. As a corollary to our main result, we obtain that any face subdivision of boundary triangles in the Sierpiński gasket graph is word-representable.     

On profinite groups with word values covered by nilpotent subgroups

Let N stand for the class of nilpotent groups or one of its well-known generalizations. For a multilinear commutator word w and a profinite group G we show that w(G) is finite-by-N if and only if the set of wvalues in G is covered by countably many finite-by-N subgroups. Earlier this was known only in the case where w = x or w = [x, y].     

On a generalization of the Hadwiger–Nelson problem

For a field F and a quadratic form Q defined on an n-dimensional vector space V over F, let QG _Q , called the quadratic graph associated to Q, be the graph with the vertex set V where vertices u,w ∈ V form an edge if and only if Q(v ? w) = 1. Quadratic graphs can be viewed as natural generalizations of the unit-distance graph featuring in the famous Hadwiger–Nelson problem. In the present paper, we will prove that for a local field F of characteristic zero, the Borel chromatic number of QG _Q is infinite if and only if Q represents zero non-trivially over F. The proof employs a recent spectral bound for the Borel chromatic number of Cayley graphs, combined with an analysis of certain oscillatory integrals over local fields. As an application, we will also answer a variant of question 525 proposed in the 22nd British Combinatorics Conference 2009 [6].     

Asymptotic energy and enstrophy concentration in solutions to the Navier–Stokes equations in <Emphasis Type="Bold">R</Emphasis><Superscript>3</Superscript>

Let A be the Stokes operator. We show as the main result of the paper that if w is a nonzero global weak solution to the Navier–Stokes equations in R ³ satisfying the strong energy inequality, then the energy of the solution w concentrates asymptotically in frequencies smaller than or equal to the finite number \({C(1/2)=\limsup\limits_{t \rightarrow \infty}||A^{1/2}w(t)||^2 / ||w(t)||^2}\) in the sense that

$\lim_{t \rightarrow \infty} || E_\lambda w(t)|| /||w(t)|| = 1$

for every λ > C(1/2), where { E _λ ; λ ≥ 0} is the resolution of identity of A. We also obtain an explicit convergence rate in the limit above and similar results for the enstrophy of w defined as ||A ^1/2 w||.

     

Strict <Emphasis Type="Italic">p</Emphasis>-negative type of a metric space

Doust and Weston (J Funct Anal 254:2336–2364, 2008) have introduced a new method called enhanced negative type for calculating a non-trivial lower bound \({\wp_{T}}\) on the supremal strict p-negative type of any given finite metric tree (T, d). In the context of finite metric trees any such lower bound \({\wp_{T} >1 }\) is deemed to be non-trivial. In this paper we refine the technique of enhanced negative type and show how it may be applied more generally to any finite metric space (X, d) that is known to have strict p-negative type for some p ≥ 0. This allows us to significantly improve the lower bounds on the supremal strict p-negative type of finite metric trees that were given in Doust and Weston (J Funct Anal 254:2336–2364, 2008, Corollary 5.5) and, moreover, leads in to one of our main results: the supremal p-negative type of a finite metric space cannot be strict. By way of application we are then able to exhibit large classes of finite metric spaces (such as finite isometric subspaces of Hadamard manifolds) that must have strict p-negative type for some p > 1. We also show that if a metric space (finite or otherwise) has p-negative type for some p > 0, then it must have strict q-negative type for all \({q \in [0, p)}\) . This generalizes Schoenberg (Ann Math 38:787–793, 1937, Theorem 2) and leads to a complete classification of the intervals on which a metric space may have strict p-negative type.     

A Note on MDS Codes, <Emphasis Type="Italic">n</Emphasis>-Arcs and Complete Designs

A generalized incidence matrix of a design over GF(q) is any matrix obtained from the (0, 1)-incidence matrix by replacing ones with nonzero elements from GF(q). The dimension d _q of a design D over GF(q) is defined as the minimum value of the q-rank of a generalized incidence matrix of D. It is proved that the dimension d _q of the complete design on n points having as blocks all w-subsets, is greater that or equal to n ? w + 1, and the equality d _q = n ? w + 1 holds if and only if there exists an [n, n ? w + 1, w] MDS code over GF(q), or equivalently, an n-arc in PG(w ? 2, q).     

Detecting fixed points of nonexpansive maps by illuminating the unit ball

We give necessary and sufficient conditions for a nonexpansive map on a finite-dimensional normed space to have a nonempty, bounded set of fixed points. Among other results we show that if f: V → V is a nonexpansive map on a finite-dimensional normed space V, then the fixed point set of f is nonempty and bounded if and only if there exist w₁,..., w _m in V such that {f(w _i ) ? w _i : i = 1,..., m} illuminates the unit ball. This yields a numerical procedure for detecting fixed points of nonexpansive maps on finite-dimensional spaces. We also discuss applications of this procedure to certain nonlinear eigenvalue problems arising in game theory and mathematical biology.     

Verbal width in anabelian groups

The class A of anabelian groups is defined as the collection of finite groups without abelian composition factors. We prove that the commutator word [x₁, x₂] and the power word x₁^p have bounded width in A when p is an odd integer. By contrast, the word x³⁰ does not have bounded width in A. On the other hand, any given word w has bounded width for those groups G ∈ A whose composition factors are sufficiently large as a function of w. In the course of the proof we establish that sufficiently large almost simple groups cannot satisfy w as a coset identity.     

On the rank of odd hyper-quasi-polynomials

Given any nonzero entire function g: ? → ?, the complex linear space F(g) consists of all entire functions f decomposable as f(z + w)g(z - w)=φ₁(z)ψ₁(w)+???+ φ_n(z)ψ_n(w) for some φ₁, ψ₁, …, φ_n, ψ_n: ? → ?. The rank of f with respect to g is defined as the minimum integer n for which such a decomposition is possible. It is proved that if g is an odd function, then the rank any function in F(g) is even.     

A note on Weyl groups and root lattices

We follow the dual approach to Coxeter systems and show for Weyl groups that a set of reflections generates the group if and only if the related sets of roots and coroots generate the root and the coroot lattices, respectively. Previously, we have proven if (W, S) is a Coxeter system of finite rank n with set of reflections T and if \(t_1, \ldots t_n \in T\) are reflections in W that generate W, then \(P:= \langle t_1, \ldots t_{n-1}\rangle \) is a parabolic subgroup of (W, S) of rank \(n-1\) (Baumeister et al. in J Group Theory 20:103–131, 2017, Theorem 1.5). Here we show if (W, S) is crystallographic as well, then all the reflections \(t \in T\) such that \(\langle P, t\rangle = W\) form a single orbit under conjugation by P.     

Neighborhood contraction in graphs

Let G be a graph and v be any vertex of G. Then the neighborhood contracted graphG_v of G, with respect to the vertex v, is the graph with vertex set V ? N(v), where two vertices u,w ∈ V ? N(v) are adjacent in G_v if either w = v and u is adjacent to any vertex of N(v) in G or u,w ? N[v] and u,w are adjacent in G. The properties of the neighborhood contracted graphs are discussed in this paper. The neighborhood contraction in some special class of graphs, the domination in a graph and the neighborhood contracted graphs are discussed in the paper.     

Free groups and ends of graphs

Let X be a connected graph. An automorphism of X is said to be parabolic if it leaves no finite subset of vertices in X invariant and fixes precisely one end of X and hyperbolic if it leaves no finite subset of vertices in X invariant and fixes precisely two ends of X. Various questions concerning dynamics of parabolic and hyperbolic automorphisms are discussed.The set of ends which are fixed by some hyperbolic element of a group G acting on X is denoted by ?(G). If G contains a hyperbolic automorphism of X and G fixes no end of X, then G contains a free subgroup F such that ?(F) is dense in ?(G) with respect to the natural topology on the ends of X.As an application we obtain the following: A group which acts transitively on a connected graph and fixes no end has a free subgroup whose directions are dense in the end boundary.     

The root class residuality of Baumslag–Solitar groups

Given a homomorphically closed root class K of groups, we find a criterion for a Baumslag–Solitar group to be a residually K-group. In particular, we establish that all Baumslag–Solitar groups are residually soluble and a Baumslag–Solitar group is residually finite soluble if and only if it is residually finite.     

Free Choosability of Outerplanar Graphs

A graph G is free (a, b)-choosable if for any vertex v with b colors assigned and for any list of colors of size a associated with each vertex \(u\ne v\), the coloring can be completed by choosing for u a subset of b colors such that adjacent vertices are colored with disjoint color sets. In this note, a necessary and sufficient condition for a cycle to be free (a, b)-choosable is given. As a corollary, we obtain almost optimal results about the free (a, b)-choosability of outerplanar graphs.     

Residual <Emphasis Type="Italic">p</Emphasis>-finiteness of generalized free products of groups

Let p be a prime number. Recall that a group G is said to be a residually finite p-group if for every non-identity element a of G there exists a homomorphism of the group G onto a finite p-group such that the image of a does not coincide with the identity. We obtain a necessary and sufficient condition for the free product of two residually finite p-groups with finite amalgamated subgroup to be a residually finite p-group. This result is a generalization of Higman’s theorem on the free product of two finite p-groups with amalgamated subgroup.     

Quasiplanar Diagrams and Slim Semimodular Lattices

For elements x and y in the (Hasse) diagram D of a finite bounded poset P, x is on the left of y, written as x λ y, if x and y are incomparable and x is on the left of all maximal chains through y. Being on the right, written as x ? y, is defined analogously. The diagram D is quasiplanar if λ and ? are transitive and for any pair (x,y) of incomparable elements, if x is on the left of some maximal chain through y, then x λ y. A planar diagram is quasiplanar, and P has a quasiplanar diagram iff its order dimension is at most 2. We are interested in diagrams only up to similarity. A finite lattice is slim if it is join-generated by the union of two chains. The main result gives a bijection between the set of (the similarity classes of) finite quasiplanar diagrams and that of (the similarity classes of) planar diagrams of finite slim semimodular lattices. This bijection allows one to describe finite posets of order dimension at most 2 by finite slim semimodular lattices, and conversely. As a corollary, we obtain that there are exactly (n?2)! quasiplanar diagrams of size n.     

How to make a linear network code (strongly) secure

A linear network code is called k-secure if it is secure even if an adversary eavesdrops at most k edges. In this paper, we show an efficient deterministic construction algorithm of a linear transformation T that transforms an (insecure) linear network code to a k-secure one for any k, and extend this algorithm to strong k-security for any k . Our algorithms run in polynomial time if k is a constant, and these time complexities are explicitly presented. We also present a concrete size of \(|\mathsf{F}|\) for strong k-security, where \(\mathsf{F}\) is the underling finite field.