Hokkaido Mathematical Journal

Vol. 46,2017

    No. 3

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    Abstract

    Decaying properties of the local energy for the dissipative wave equations with the Dirichlet boundary conditions in exterior domains are discussed. For the dissipation coefficient, natural conditions ensuring that waves trapped by obstacles may lose their energy are considered. Under this setting, two sufficient conditions for getting the decay estimates for the energy in bounded regions (i.e. the local energy) are given. These conditions bring some relaxation on classes of the dissipation coefficient which uniformly decaying estimates for the local energy hold. Further, decaying properties of the total energy are also discussed.

    MSC(Primary) 35L05
    MSC(Secondary) 35B40;
    Uncontrolled Keywords Dissipative wave equations; exterior problems; local energy decay; total energy decay; non-compactly supported initial data;

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    Abstract

    We give criteria for singularities of spacelike constant mean curvature surfaces in 3-dimensional de Sitter and anti-de Sitter spaces constructed by the DPW method, which is a generalized Weierstrass representation. We also construct some examples of spacelike CMC surfaces, including analogs of Smyth surfaces with singularities, using appropriate models to visualize them.

    MSC(Primary) 53B30
    MSC(Secondary) 53A10;
    Uncontrolled Keywords differential geometry; surface theory; integrable systems;
    SHEN, Conghui; XU, Jingshi;
    A vector-valued estimate of multilinear Calderón-Zygmund operators in Herz-Morrey spaces with variable exponents.
    Hokkaido Mathematical Journal, 46 (2017) pp.351-380

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    In this paper, we obtain a vector valued inequality of multilinear Calderón-Zygmund operators on products of Herz-Morrey spaces with variable exponents.

    MSC(Primary) 42B25
    MSC(Secondary) 42B35;
    Uncontrolled Keywords multilinear Calderón-Zygmund operator; variable exponent; Herz-Morrey space; vector valued estimate;
    BAKHTYIARI, M.; NIKMEHR, M. J.; NIKANDISH, R.;
    The extended zero-divisor graph of a commutative ring I.
    Hokkaido Mathematical Journal, 46 (2017) pp.381-393

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    Abstract

    Let $R$ be a commutative ring with identity, and let $Z(R)$ be the set of zero-divisors of $R$. The extended zero-divisor graph of $R$ is the undirected (simple) graph $\Gamma'(R)$ with the vertex set $Z(R)^*=Z(R)\setminus\{0\}$, and two distinct vertices $x$ and $y$ are adjacent if and only if either $Rx\cap \mathrm{Ann}(y)\neq (0)$ or $Ry\cap \mathrm{Ann}(x)\neq (0)$. It follows that the zero-divisor graph $\Gamma(R)$ is a subgraph of $\Gamma'(R)$. It is proved that $\Gamma'(R)$ is connected with diameter at most two and with girth at most four, if $\Gamma'(R)$ contains a cycle. Moreover, we characterize all rings whose extended zero-divisor graphs are complete or star. Furthermore, we study the affinity between extended zero-divisor graph and zero-divisor graph associated with a commutative ring. For instance, for a non-reduced ring $R$, it is proved that the extended zero-divisor graph and the zero-divisor graph of $R$ are identical to the join of a complete graph and a null graph if and only if $ann_R(Z(R))$ is a prime ideal.

    MSC(Primary) 13A15
    MSC(Secondary) 13B99; 05C99;
    Uncontrolled Keywords Extended zero-divisor graph; Zero-divisor graph; Complete graph;
    BAKHTYIARI, M.; NIKMEHR, M. J.; NIKANDISH, R.;
    The extended zero-divisor graph of a commutative ring II.
    Hokkaido Mathematical Journal, 46 (2017) pp.395-406

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    Abstract

    Let $R$ be a commutative ring with identity, and let $Z(R)$ be the set of zero-divisors of $R$. The extended zero-divisor graph of $R$ is the undirected (simple) graph $\Gamma'(R)$ with the vertex set $Z(R)^*=Z(R)\setminus\{0\}$, and two distinct vertices $x$ and $y$ are adjacent if and only if either $Rx\cap \mathrm{Ann}(y)\neq (0)$ or $Ry\cap \mathrm{Ann}(x)\neq (0)$. In this paper, we continue our study of the extended zero-divisor graph of a commutative ring that was introduced in [4]. We show that the extended zero-divisor graph associated with an Artinian ring is weakly perfect, i.e., its vertex chromatic number equals its clique number. Furthermore, we classify all rings whose extended zero-divisor graphs are planar.

    MSC(Primary) 13B99
    MSC(Secondary) 05C10; 05C69;
    Uncontrolled Keywords Extended zero-divisor graph; Clique number; Chromatic number; Planar graph;
    FUKUMA, Yoshiaki; ITO, Kazuhisa;
    On the class of projective surfaces of general type.
    Hokkaido Mathematical Journal, 46 (2017) pp.407-422

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    Abstract

    Let $S$ be a smooth complex projective surface of general type, $H$ be a very ample divisor on $S$ and $m(S,H)$ be the class of $(S,H)$. In this paper, we study a lower bound for $m(S,H)-3H^2$ and we improve an inequality obtained by Lanteri. We also study $(S,H)$ with each value of $m(S,H)-3H^2$ and exhibit some examples.

    MSC(Primary) 14C20
    MSC(Secondary) 14J29;
    Uncontrolled Keywords Class; surfaces of general type; very ample divisor; sectional genus; $\Delta$-genus;
    WADA, Kazuyuki;
    Spectral analysis of a massless charged scalar field with cutoffs.
    Hokkaido Mathematical Journal, 46 (2017) pp.423-471

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    Abstract

    A quantum system of a massless charged scalar field with a self-interaction is investigated. By introducing a spacial cut-off function, a Hamiltonian of the quantum system is realized as a linear operator on a boson Fock space. Under certain conditions, it is proven that the Hamiltonian is bounded below, self-adjoint and has a ground state for an arbitrary coupling constant. Moreover the Hamiltonian strongly commutes with the total charge operator. This fact implies that the state space of the charged scalar field is decomposed into the infinite direct sum of fixed total charge spaces. A total charge of an eigenstate is discussed.

    MSC(Primary) 47B25
    MSC(Secondary) 81Q10;
    Uncontrolled Keywords Quantum field theory; Charged scalar field; Spectral analysis;
    MATSUMOTO, Shigenori;
    Kinematic expansive suspensions of irrational rotations on the circle.
    Hokkaido Mathematical Journal, 46 (2017) pp.473-485

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    We shall show that the rotation of some irrational rotation number on the circle admits suspensions which are kinematic expansive.

    MSC(Primary) 37E10
    MSC(Secondary)
    Uncontrolled Keywords kinematic expansive flows; suspensions; irrational rotations;
    BEDDANI, Hamid; HAMANI, Karima;
    Growth of meromorphic solutions of some linear differential equations.
    Hokkaido Mathematical Journal, 46 (2017) pp.487-512

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    In this paper, we investigate the order and the hyper-order of meromorphic solutions of the linear differential equation \begin{equation*} f^{(k)}+\sum^{k-1}_{j=1}(D_{j}+B_{j}e^{P_{j}(z) })f^{(j)}+( D_{0}+A_{1}e^{Q_{1}( z)}+A_{2}e^{Q_{2}( z) }) f=0, \end{equation*} where $k\geq 2$ is an integer, $Q_{1}(z),Q_{2}(z)$, $P_{j}(z) $ $(j=1, \dots ,k-1)$ are nonconstant polynomials and $A_{s}(z)$ $(\not\equiv 0)$ $(s=1,2)$, $B_{j}( z)$ $(\not\equiv 0)$ $(j=1, \dots ,k-1)$, $D_{m}(z)$ $(m=0,1, \dots ,k-1)$ are meromorphic functions. Under some conditions, we prove that every meromorphic solution $f$ $(\not\equiv 0)$ of the above equation is of infinite order and we give an estimate of its hyper-order. Furthermore, we obtain a result about the exponent of convergence and the hyper-exponent of convergence of a sequence of zeros and distinct zeros of $f-\varphi$, where $\varphi$ $(\not\equiv 0)$ is a meromorphic function and $f$ $(\not\equiv 0)$ is a meromorphic solution of the above equation.

    MSC(Primary) 34M10
    MSC(Secondary) 30D35;
    Uncontrolled Keywords Linear Differential Equation; Meromorphic function; Hyper-order; Exponent of convergence; hyper-exponent of convergence;