On dihedrants admitting arc-regular group actions

We consider Cayley graphs Γ over dihedral groups, dihedrants for short, which admit an automorphism group G acting regularly on the arc set of Γ. We prove that, if D _2n ≤G≤Aut(Γ) is a regular dihedral subgroup of G of order 2n such that any of its index 2 cyclic subgroups is core-free in G, then Γ belongs to the family of graphs of the form \((K_{n_{1}}\otimes\cdots\otimes K_{n_{\ell}})[K_{m}^{\mathrm{c}}]\), where 2n=n ₁???n _? m, and the numbers n _i are pairwise coprime. Applications to 1-regular dihedrants and Cayley maps on dihedral groups are also given.     

A new linear quotient of C 4 admitting a symplectic resolution

We show that the quotient C ⁴/G admits a symplectic resolution for ${G = Q_8 \times_{{\bf Z}/2} D_8 < {\sf Sp}_4({\bf C})}$ . Here Q ₈ is the quaternionic group of order eight and D ₈ is the dihedral group of order eight, and G is the quotient of their direct product which identifies the nontrivial central elements ?Id of each. It is equipped with the tensor product representation ${{\bf C}^2 \boxtimes {\bf C}^2 \cong {\bf C}^4}$ . This group is also naturally a subgroup of the wreath product group ${Q_8^2 \rtimes S_2 < {\sf Sp}_4({\bf C})}$ . We compute the singular locus of the family of commutative spherical symplectic reflection algebras deforming C ⁴/G. We also discuss preliminary investigations on the more general question of classifying linear quotients V/G admitting symplectic resolutions.     

Strong Asymptotics for Bergman Polynomials over Domains with Corners and Applications

Let G be a bounded simply-connected domain in the complex plane ?, whose boundary Γ:=?G is a Jordan curve, and let $\{p_{n}\}_{n=0}^{\infty}$ denote the sequence of Bergman polynomials of G. This is defined as the unique sequence $$p_n(z) = \lambda_n z^n+\cdots, \quad \lambda_n>0,\ n=0,1,2,\ldots, $$ of polynomials that are orthonormal with respect to the inner product $$\langle f,g\rangle := \int_G f(z) \overline{g(z)} \,dA(z), $$ where dA stands for the area measure. We establish the strong asymptotics for p _n and λ _n , n∈?, under the assumption that Γ is piecewise analytic. This complements an investigation started in 1923 by T. Carleman, who derived the strong asymptotics for Γ analytic, and carried over by P.K. Suetin in the 1960s, who established them for smooth Γ. In order to do so, we use a new approach based on tools from quasiconformal mapping theory. The impact of the resulting theory is demonstrated in a number of applications, varying from coefficient estimates in the well-known class Σ of univalent functions and a connection with operator theory, to the computation of capacities and a reconstruction algorithm from moments.     

The Dual Group of a Dense Subgroup

Throughout this abstract, G is a topological Abelian group and $\hat G$ is the space of continuous homomorphisms from G into the circle group ${\mathbb{T}}$ in the compact-open topology. A dense subgroup D of G is said to determine G if the (necessarily continuous) surjective isomorphism $\hat G \to \hat D$ given by $h \mapsto h\left| D \right.$ is a homeomorphism, and G is determined if each dense subgroup of G determines G. The principal result in this area, obtained independently by L. Außenhofer and M. J. Chasco, is the following: Every metrizable group is determined. The authors offer several related results, including these. 1. There are (many) nonmetrizable, noncompact, determined groups. 2. If the dense subgroup D _i determines G _i with G _i compact, then $ \oplus _i D_i $ determines Π_i G _i. In particular, if each G _i is compact then $ \oplus _i G_i $ determines Π_i G _i. 3. Let G be a locally bounded group and let G ⁺ denote G with its Bohr topology. Then G is determined if and only if G ⁺ is determined. 4. Let non $\left( {\mathcal{N}} \right)$ be the least cardinal κ such that some $X \subseteq {\mathbb{T}}$ of cardinality κ has positive outer measure. No compact G with $w\left( G \right) \geqslant non\left( {\mathcal{N}} \right)$ is determined; thus if $\left( {\mathcal{N}} \right) = {\mathfrak{N}}_1 $ (in particular if CH holds), an infinite compact group G is determined if and only if w(G) = ω. Question. Is there in ZFC a cardinal κ such that a compact group G is determined if and only if w(G) < κ? Is $\kappa = non\left( {\mathcal{N}} \right)?\kappa = {\mathfrak{N}}_1 ?$      

Estimate of a polynomial in generalized exponential functions in the convex hull of two domains

Пусть $$f_n (z) = \exp \{ \lambda _n z\} [1 + \psi _n (z)], n \geqq 1$$ гдеψ _n (z) — регулярны в н екоторой односвязно й областиS, λ _n — нули целой функц ии экспоненциальног о ростаL(λ) с индикатрис ой ростаh(?), причем $$|L\prime (\lambda _n )| > C(\delta )\exp \{ [h(\varphi _n ) - \varepsilon ]|\lambda _n |\} \varphi _n = \arg \lambda _n , \forall \varepsilon > 0$$ . Предположим, что на лю бом компактеK?S $$|\psi _n (z)|< Aq^{|\lambda |_n } , a< q< 1, n \geqq 1$$ гдеA иq зависит только отK. Обозначим через \(\bar D\) со пряженную диаграмму функцииL(λ), через \(\bar D_\alpha \) — смещение. \(\bar D\) на векторα. Рассмотр им множестваD ₁ иD ₂ так ие, чтоD ₁ иD ₂ и их вьшуклая обо лочкаE принадлежатS. Пусть \(\bar D_{\alpha _1 } \subset D_1 , \bar D_{\alpha _2 } \subset D_2 \) Доказывается, что сущ ествует некоторая об ластьG?E такая, что \(\mathop \cup \limits_{\alpha \in [\alpha _1 ,\alpha _2 ]} \bar D_\alpha \subset G\) и дляz∈G верна оценка $$\sum\limits_{v = 1}^n {|a_v f_v (z)|} \leqq B\max (M_1 ,M_2 ), M_j = \mathop {\max }\limits_{t \in \bar D_j } |\sum\limits_{v = 1}^n {a_v f_v (t)} |$$ , где константаB не зав исит от {a _v }.     

Completely regular clique graphs

Let Γ=(X,R) be a connected graph. Then Γ is said to be a completely regular clique graph of parameters (s,c) with s≥1 and c≥1, if there is a collection \(\mathcal{C}\) of completely regular cliques of size s+1 such that every edge is contained in exactly c members of  \(\mathcal{C}\) . In this paper, we show that the parameters of \(C\in\mathcal{C}\) as a completely regular code do not depend on \(C\in\mathcal{C}\) . As a by-product we have that all completely regular clique graphs are distance-regular whenever \(\mathcal {C}\) consists of edges. We investigate the case when Γ is distance-regular, and show that Γ is a completely regular clique graph if and only if it is a bipartite half of a distance-semiregular graph.     

Untangling a Planar Graph

A straight-line drawing δ of a planar graph G need not be plane but can be made so by untangling it, that is, by moving some of the vertices of G. Let shift(G,δ) denote the minimum number of vertices that need to be moved to untangle δ. We show that shift(G,δ) is NP-hard to compute and to approximate. Our hardness results extend to a version of 1BendPointSetEmbeddability, a well-known graph-drawing problem. Further we define fix(G,δ)=n?shift(G,δ) to be the maximum number of vertices of a planar n-vertex graph G that can be fixed when untangling δ. We give an algorithm that fixes at least $\sqrt{((\log n)-1)/\log\log n}$ vertices when untangling a drawing of an n-vertex graph G. If G is outerplanar, the same algorithm fixes at least $\sqrt{n/2}$ vertices. On the other hand, we construct, for arbitrarily large n, an n-vertex planar graph G and a drawing δ _G of G with $\ensuremath {\mathrm {fix}}(G,\delta_{G})\leq \sqrt{n-2}+1$ and an n-vertex outerplanar graph H and a drawing δ _H of H with $\ensuremath {\mathrm {fix}}(H,\delta_{H})\leq2\sqrt{n-1}+1$ . Thus our algorithm is asymptotically worst-case optimal for outerplanar graphs.     

Rainbow Connection Number, Bridges and Radius

Let G be a connected graph. The notion of rainbow connection number rc(G) of a graph G was introduced by Chartrand et al. (Math Bohem 133:85–98, 2008). Basavaraju et al. (arXiv:1011.0620v1 [math.CO], 2010) proved that for every bridgeless graph G with radius r, ${rc(G)\leq r(r+2)}$ and the bound is tight. In this paper, we show that for a connected graph G with radius r and center vertex u, if we let D ^r  = {u}, then G has r?1 connected dominating sets ${ D^{r-1}, D^{r-2},\ldots, D^{1}}$ such that ${D^{r} \subset D^{r-1} \subset D^{r-2} \cdots\subset D^{1} \subset D^{0}=V(G)}$ and ${rc(G)\leq \sum_{i=1}^{r} \max \{2i+1,b_i\}}$ , where b _i is the number of bridges in E[D ⁱ , N(D ⁱ )] for ${1\leq i \leq r}$ . From the result, we can get that if ${b_i\leq 2i+1}$ for all ${1\leq i\leq r}$ , then ${rc(G)\leq \sum_{i=1}^{r}(2i+1)= r(r+2)}$ ; if b _i  > 2i + 1 for all ${1\leq i\leq r}$ , then ${rc(G)= \sum_{i=1}^{r}b_i}$ , the number of bridges of G. This generalizes the result of Basavaraju et al. In addition, an example is given to show that there exist infinitely graphs with bridges whose rc(G) is only dependent on the radius of G, and another example is given to show that there exist infinitely graphs with bridges whose rc(G) is only dependent on the number of bridges in G.     

An Upper Bound for the Total Restrained Domination Number of Graphs

Let G be a graph with vertex set V. A set ${D \subseteq V}$ is a total restrained dominating set of G if every vertex in V has a neighbor in D and every vertex in ${V \setminus D}$ has a neighbor in ${V \setminus D}$ . The minimum cardinality of a total restrained dominating set of G is called the total restrained domination number of G, and is denoted by γ _tr (G). In this paper, we prove that if G is a connected graph of order n ≥ 4 and minimum degree at least two, then ${\gamma_{tr}(G) \leq n-\sqrt[3]{n \over 4}}$ .     

Every group is a maximal subgroup of a naturally occurring free idempotent generated semigroup

The study of the free idempotent generated semigroup IG(E) over a biordered set E has recently received a deal of attention. Let G be a group, let \(n\in\mathbb{N}\) with n≥3 and let E be the biordered set of idempotents of the wreath product \(G\wr \mathcal{T}_{n}\) . We show, in a transparent way, that for e∈E lying in the minimal ideal of \(G\wr\mathcal{T}_{n}\) , the maximal subgroup of e in IG(E) is isomorphic to G. It is known that \(G\wr\mathcal{T}_{n}\) is the endomorphism monoid End F _n (G) of the rank n free G-act F _n (G). Our work is therefore analogous to that of Brittenham, Margolis and Meakin for rank 1 idempotents in full linear monoids. As a corollary we obtain the result of Gray and Ru?kuc that any group can occur as a maximal subgroup of some free idempotent generated semigroup. Unlike their proof, ours involves a natural biordered set and very little machinery.     

On Some Ramsey Numbers for Quadrilaterals Versus Wheels

For given graphs G ₁ and G ₂, the Ramsey number R(G ₁, G ₂) is the least integer n such that every 2-coloring of the edges of K _n contains a subgraph isomorphic to G ₁ in the first color or a subgraph isomorphic to G ₂ in the second color. Surahmat et al. proved that the Ramsey number ${R(C_4, W_n) \leq n + \lceil (n-1)/3\rceil}$ . By using asymptotic methods one can obtain the following property: ${R(C_4, W_n) \leq n + \sqrt{n}+o(1)}$ . In this paper we show that in fact ${R(C_4, W_n) \leq n + \sqrt{n-2}+1}$ for n ≥ 11. Moreover, by modification of the Erd?s-Rényi graph we obtain an exact value ${R(C_4, W_{q^2+1}) = q^2 + q + 1}$ with q ≥ 4 being a prime power. In addition, we provide exact values for Ramsey numbers R(C ₄, W _n ) for 14 ≤ n ≤ 17.     

Note on Group Distance Magic Graphs G[C 4]

A group distance magic labeling or a ${\mathcal{G}}$ -distance magic labeling of a graph G =  (V, E) with ${|V | = n}$ is a bijection f from V to an Abelian group ${\mathcal{G}}$ of order n such that the weight ${w(x) = \sum_{y\in N_G(x)}f(y)}$ of every vertex ${x \in V}$ is equal to the same element ${\mu \in \mathcal{G}}$ , called the magic constant. In this paper we will show that if G is a graph of order n =  2 ^p (2k + 1) for some natural numbers p, k such that ${\deg(v)\equiv c \mod {2^{p+1}}}$ for some constant c for any ${v \in V(G)}$ , then there exists a ${\mathcal{G}}$ -distance magic labeling for any Abelian group ${\mathcal{G}}$ of order 4n for the composition G[C ₄]. Moreover we prove that if ${\mathcal{G}}$ is an arbitrary Abelian group of order 4n such that ${\mathcal{G} \cong \mathbb{Z}_2 \times\mathbb{Z}_2 \times \mathcal{A}}$ for some Abelian group ${\mathcal{A}}$ of order n, then there exists a ${\mathcal{G}}$ -distance magic labeling for any graph G[C ₄], where G is a graph of order n and n is an arbitrary natural number.     

Pairs of Disjoint Dominating Sets and the Minimum Degree of Graphs

For a connected graph G of order n and minimum degree δ we prove the existence of two disjoint dominating sets D ₁ and D ₂ such that, if δ ≥ 2, then ${|D_1\cup D_2|\leq \frac{6}{7}n}$ unless G = C ₄, and, if δ ≥ 5, then ${|D_1\cup D_2|\leq 2\frac{1+\ln(\delta+1)}{\delta+1}n}$ . While for the first estimate there are exactly six extremal graphs which are all of order 7, the second estimate is asymptotically best-possible.     

Arc-transitive Dihedrants of Odd Prime-power Order

Let G be a finite group with identity element 1, and S be a subset of G such that ${1 \notin S}$ and S = S ^?1. The Cayley graph Cay(G, S) has vertex set G, and x, y in G are adjacent if and only if ${xy^{-1} \in S}$ . In this paper we classify the connected, arc-transitive Cayley graphs ${{\rm Cay}(D_{2p^n}, S),}$ where ${D_{2p^n}}$ is the dihedral group of order 2p ⁿ , p is an odd prime.     

A relationship between bounds on the sum of squares of degrees of a graph

LetG = (V, E) be a simple graph withn vertices and e edges. Letdi be the degree of the ith vertex v_i ∈ V andm _i the average of the degrees of the vertices adjacent to vertexv _i ∈ V. It is known by Caen [1] and Das [2] that $\frac{{4e^2 }}{n} \leqslant d_1^2 + ... + d_n^2 \leqslant e max \{ d_j + m_j |v_j \in V\} \leqslant e\left( {\frac{{2e}}{{n - 1}} + n - 2} \right)$ . In general, the equalities do not hold in above inequality. It is shown that a graphG is regular if and only if $\frac{{4e^2 }}{n} = d_1^2 + ... + d_n^2 $ . In fact, it is shown a little bit more strong result that a graphG is regular if and only if $\frac{{4e^2 }}{n} = d_1^2 + ... + d_n^2 = e max \{ d_j + m_j |v_j \in V\} $ . For a graphG withn < 2 vertices, it is shown that G is a complete graphK _n if and only if $\frac{{4e^2 }}{n} = d_1^2 + ... + d_n^2 = e max \{ d_j + m_j |v_j \in V\} = e\left( {\frac{{2e}}{{n - 1}} + n - 2} \right)$ .     

K(π, 1) and word problems for infinite type Artin–Tits groups, and applications to virtual braid groups

Let Γ be a Coxeter graph, let (W, S) be its associated Coxeter system, and let (A, Σ) be its associated Artin–Tits system. We regard W as a reflection group acting on a real vector space V. Let I be the Tits cone, and let E _Γ be the complement in I + iV of the reflecting hyperplanes. Recall that Salvetti, Charney and Davis have constructed a simplicial complex Ω(Γ) having the same homotopy type as E _Γ. We observe that, if ${T \subset S}$ , then Ω(Γ _T ) naturally embeds into Ω (Γ). We prove that this embedding admits a retraction ${\pi_T: \Omega(\Gamma) \to \Omega (\Gamma_T)}$ , and we deduce several topological and combinatorial results on parabolic subgroups of A. From a family ${\mathcal{S}}$ of subsets of S having certain properties, we construct a cube complex Φ, we show that Φ has the same homotopy type as the universal cover of E _Γ, and we prove that Φ is CAT(0) if and only if ${\mathcal{S}}$ is a flag complex. We say that ${X \subset S}$ is free of infinity if Γ _X has no edge labeled by ∞. We show that, if ${E_{\Gamma_X}}$ is aspherical and A _X has a solution to the word problem for all ${X \subset S}$ free of infinity, then E _Γ is aspherical and A has a solution to the word problem. We apply these results to the virtual braid group VB _n . In particular, we give a solution to the word problem in VB _n , and we prove that the virtual cohomological dimension of VB _n is n?1.     

Big elements in irreducible linear groups

Let V be a linear space over a field K of dimension n > 1, and let \({G \leq {\rm GL}(V)}\) be an irreducible linear group. In this paper we prove that the group G contains an element g such that rank \({(g - \alpha E_{n}) \geq \frac{n}{2}}\) for every \({\alpha \in K}\) , where E _n is the identity operator on V. This estimate is sharp for any \({n = 2^{m}}\) . The existence of such an element implies that the conjugacy class of G in GL(V) intersects the big Bruhat cell \({B\dot{w}_{0}B}\) of GL(V) non-trivially (here B is a fixed Borel subgroup of G). The latter fact is equivalent to the existence of a complete flag \({\mathfrak{F}}\) such that the flags \({g(\mathfrak{F}), \mathfrak{F}}\) are in general position for some g ∈ G.     

On the Edge Connectivity of Direct Products with Dense Graphs

Let ${\kappa^{\prime}(G)}$ be the edge connectivity of G and G × H the direct product of G and H. Let H be any graph with minimal degree ${\delta(H)>|V(H)|/2}$ . We prove that for any graph ${G, \kappa^{\prime}(G\times H)=\textup{min}\{2\kappa^{\prime}(G)|E(H)|,\delta(G)\delta(H)\}}$ . In addition, the structure of minimum edge cuts is described. As an application, we present a necessary and sufficient condition for G × K _n (n ≥ 3) to be super edge connected.     

Exponent of a finite group with a four-group of automorphisms

Let A be a group isomorphic with either S ₄, the symmetric group on four symbols, or D ₈, the dihedral group of order 8. Let V be a normal four-subgroup of A and ?? an involution in ${A\setminus V}$ . Suppose that A acts on a finite group G in such a manner that C _G (V)?=?1 and C _G (??) has exponent e. We show that if ${A\cong S_4}$ then the exponent of G is e-bounded and if ${A\cong D_8}$ then the exponent of the derived group G?? is e-bounded. This work was motivated by recent results on the exponent of a finite group admitting an action by a Frobenius group of automorphisms.     

2-Arc-transitive cyclic covers of K n,n −nK 2

Du et al. (in J. Comb. Theory B 74:276–290, 1998 and J. Comb. Theory B 93:73–93, 2005), classified regular covers of complete graph whose fiber-preserving automorphism group acts 2-arc-transitively, and whose covering transformation group is either cyclic or isomorphic to $\mathbb{Z}_{p}^{2}$ or $\mathbb{Z}_{p}^{3}$ with p a prime. In this paper, a complete classification is achieved of all the regular covers of bipartite complete graphs minus a matching K _n,n ?nK ₂ with cyclic covering transformation groups, whose fiber-preserving automorphism groups act 2-arc-transitively.