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Karen Yagdjian

Publications and source records attributed to Karen Yagdjian.

29 records · Page 2Linked to original sources
arXiv2014

Integral transform approach to solving Klein-Gordon equation with variable coefficients

In this paper we describe the integral transform that allows to write solutions of one partial differential equation via solution of another one. This transform was suggested by the author in the case when the last equation is a wave equation, and then it was used to investigate several well-known equations such as Tricomi-type equation, the Klein-Gordon equation in the de~Sitter and Einstein-de~Sitter spacetimes. A generalization given in this paper allows us to consider also the Klein-Gordon equations with coefficients depending on the spatial variables.

math.AP↗
arXiv2014

Global solutions for semilinear Klein-Gordon equations in FLRW spacetimes

We consider waves, which obey the semilinear Klein-Gordon equation, propagating in the Friedmann-Lemaitre-Robertson-Walker spacetimes. The equations in the de Sitter and Einstein-de Sitter spacetimes are the important particular cases. We show the global in time existence in the energy class of solutions of the Cauchy problem.

math.AP↗
arXiv2013

Semilinear Hyperbolic Equations in Curved Spacetime

This is a survey of the author's recent work rather than a broad survey of the literature. The survey is concerned with the global in time solutions of the Cauchy problem for matter waves propagating in the curved spacetimes, which can be, in particular, modeled by cosmological models. We examine the global in time solutions of some class of semililear hyperbolic equations, such as the Klein-Gordon equation, which includes the Higgs boson equation in the Minkowski spacetime, de Sitter spacetime, and Einstein & de Sitter spacetime. The crucial tool for the obtaining those results is a new approach suggested by the author based on the integral transform with the kernel containing the hypergeometric function.\\ {\bf Mathematics Subject Classification (2010):} Primary 35L71, 35L53; Secondary 81T20, 35C15.\\ {\bf Keywords:} \small {de Sitter spacetime; Klein-Gordon equation; Global solutions; Huygens' principle; Higuchi bound}

math.AP↗
arXiv2012

Huygens' Principle for the Klein-Gordon equation in the de Sitter spacetime

In this article we prove that the Klein-Gordon equation in the de Sitter spacetime obeys the Huygens' principle only if the physical mass $m$ of the scalar field and the dimension $n\geq 2$ of the spatial variable are tied by the equation $m^2=(n^2-1)/4 $. Moreover, we define the incomplete Huygens' principle, which is the Huygens' principle restricted to the vanishing second initial datum, and then reveal that the massless scalar field in the de Sitter spacetime obeys the incomplete Huygens' principle and does not obey the Huygens' principle, for the dimensions $n=1,3$, only. Thus, in the de Sitter spacetime the existence of two different scalar fields (in fact, with m=0 and $m^2=(n^2-1)/4 $), which obey incomplete Huygens' principle, is equivalent to the condition $n=3$ (in fact, the spatial dimension of the physical world). For $n=3$ these two values of the mass are the endpoints of the so-called in quantum field theory the Higuchi bound. The value $m^2=(n^2-1)/4 $ of the physical mass allows us also to obtain complete asymptotic expansion of the solution for the large time. Keywords: Huygens' Principle; Klein-Gordon Equation; de Sitter spacetime; Higuchi Bound

math-ph↗
arXiv2011

Parametric Resonance in Wave Maps

In this note we concern with the wave maps from the Lorentzian manifold with the periodic in time metric into the Riemannian manifold, which belongs to the one-parameter family of Riemannian manifolds. That family contains as a special case the Poincare upper half-plane model. Our interest to such maps is motivated with some particular type of the Robertson-Walker spacetime arising in the cosmology. We show that small periodic in time perturbation of the Minkowski metric generates parametric resonance phenomenon. We prove that, the global in time solvability in the neighborhood of constant solutions is not a stable property of the wave maps.

math.AP↗
arXiv2010

On the global solutions of the Higgs boson equation

In this article we study global in time (not necessarily small) solutions of the equation for the Higgs boson in the Minkowski and in the de Sitter spacetimes. We reveal some qualitative behavior of the global solutions. In particular, we formulate sufficient conditions for the existence of the zeros of global solutions in the interior of their supports, and, consequently, for the creation of the so-called bubbles, which have been studied in particle physics and inflationary cosmology. We also give some sufficient conditions for the global solution to be an oscillatory in time solution.

math.AP↗
arXiv2010

A Note on Wave Equation in Einstein & de Sitter Spacetime

We consider the wave propagating in the Einstein & de Sitter spacetime. The covariant d'Alembert's operator in the Einstein & de Sitter spacetime belongs to the family of the non-Fuchsian partial differential operators. We introduce the initial value problem for this equation and give the explicit representation formulas for the solutions. We also show the $L^p - L^q$ estimates for solutions.

math-ph↗
arXiv2010

Fundamental Solutions for Hyperbolic Operators with Variable Coefficients

In this article we describe the novel method to construct fundamental solutions for operators with variable coefficients. That method was introduced in "A note on the fundamental solution for the Tricomi-type equation in the hyperbolic domain"(J. Differential Equations,206(2004)227--252) to study the Tricomi-type equation. More precisely, the new integral operator is suggested which transforms the family of the fundamental solutions of the Cauchy problem for the equation with the constant coefficients to the fundamental solutions for the operators with variable coefficients.

math.AP↗
arXiv2009

The semilinear Klein-Gordon equation in de Sitter spacetime

In this article we study the blow-up phenomena for the solutions of the semilinear Klein-Gordon equation $\Box_g ϕ-m^2 ϕ= -|ϕ|^p $ with the small mass $m \le n/2$ in de Sitter space-time with the metric $g$. We prove that for every $p>1$ the large energy solution blows up, while for the small energy solutions we give a borderline $p=p(m,n)$ for the global in time existence. The consideration is based on the representation formulas for the solution of the Cauchy problem and on some generalizations of the Kato's lemma.

math-ph↗
arXiv2007

Fundamental Solutions for Wave Equation in de Sitter Model of Universe

In this article we construct the fundamental solutions for the wave equation arising in the de Sitter model of the universe. We use the fundamental solutions to represent solutions of the Cauchy problem and to prove the $L^p-L^q$-decay estimates for the solutions of the equation with and without a source term.

math.AP↗