Searcharxiv⌕ Search

arXiv subjects

Masahiro Yamamoto

Publications and source records attributed to Masahiro Yamamoto.

At least 109 records · Page 6Linked to original sources
arXiv2017

Carleman estimate and application to an inverse source problem for a viscoelasticity model in anisotropic case

We consider an anisotropic hyperbolic equation with memory term: $$ \partial_t^2 u(x,t) = \sum_{i,j=1}^n \partial_i(a_{ij}(x)\partial_ju) + \int^t_0 \sum_{| α| \le 2} b_α(x,t,η)\partial_x^αu(x,η) dη+ F(x,t) $$ for $x \in Ω$ and $t\in (0,T)$ or $\in (-T,T)$, which is a model equation for viscoelasticity. First we establish a Carleman estimate for this equation with overdetermining boundary data on a suitable lateral subboundary $Γ\times (-T,T)$. Second we apply the Carleman estimate to establish a both-sided estimate of $| u(\cdot,0)|_{H^3(Ω)}$ by $\partial_νu|_{Γ\times (0,T)}$ under the assumption that $\partial_tu(\cdot,0) = 0$ and $T>0$ is sufficiently large, $Γ\subset \partialΩ$ satisfies some geometric condition. Such an estimate is a kind of observability inequality and related to the exact controllability. Finally we apply the Carleman estimate to an inverse source problem of determining a spatial varying factor in $F(x,t)$ and we establish a both-sided Lipschitz stability estimate.

math.AP↗
arXiv2016

Weak unique continuation property and a related inverse source problem for time-fractional diffusion-advection equations

In this paper, we first establish a weak unique continuation property for time-fractional diffusion-advection equations. The proof is mainly based on the Laplace transform and the unique continuation properties for elliptic and parabolic equations. The result is weaker than its parabolic counterpart in the sense that we additionally impose the homogeneous boundary condition. As a direct application, we prove the uniqueness for an inverse problem on determining the spatial component in the source term in by interior measurements. Numerically, we reformulate our inverse source problem as an optimization problem, and propose an iteration thresholding algorithm. Finally, several numerical experiments are presented to show the accuracy and efficiency of the algorithm.

math.AP↗
arXiv2016

Global uniqueness in an inverse problem for time fractional diffusion equations

Given $(M,g)$, a compact connected Riemannian manifold of dimension $d \geq 2$, with boundary $\partial M$, we consider an initial boundary value problem for a fractional diffusion equation on $(0,T) \times M$, $T>0$, with time-fractional Caputo derivative of order $α\in (0,1) \cup (1,2)$. We prove uniqueness in the inverse problem of determining the smooth manifold $(M,g)$ (up to an isometry), and various time-independent smooth coefficients appearing in this equation, from measurements of the solution on a subset of $\partial M$ at fixed time. In the "flat" case where $M$ is a compact subset of $\mathbb R^d$, two out the three coefficients $ρ$ (weight), $a$ (conductivity) and $q$ (potential) appearing in the equation $ρ\partial_t^αu-\textrm{div}(a \nabla u)+ q u=0$ on $(0,T)\times Ω$ are recovered simultaneously.

math.AP↗
arXiv2015

On existence and uniqueness of solutions for semilinear fractional wave equations

Let $Ω$ be a $\mathcal C^2$-bounded domain of $\mathbb R^d$, $d=2,3$, and fix $Q=(0,T)\timesΩ$ with $T\in(0,+\infty]$. In the present paper we consider a Dirichlet initial-boundary value problem associated to the semilinear fractional wave equation $\partial_t^αu+\mathcal A u=f_b(u)$ in $Q$ where $1<α<2$, $\partial_t^α$ corresponds to the Caputo fractional derivative of order $α$, $\mathcal A$ is an elliptic operator and the nonlinearity $f_b\in \mathcal C^1( \mathbb R)$ satisfies $f_b(0)=0$ and $|f_b'(u)|\leq C|u|^{b-1}$ for some $b>1$. We first provide a definition of local weak solutions of this problem by applying some properties of the associated linear equation $\partial_t^αu+\mathcal A u=f(t,x)$ in $Q$. Then, we prove existence of local solutions of the semilinear fractional wave equation for some suitable values of $b>1$. Moreover, we obtain an explicit dependence of the time of existence of solutions with respect to the initial data that allows longer time of existence for small initial data.

math.AP↗
arXiv2015

Theoretical stability and numerical reconstruction for an inverse source problem for hyperbolic equations

In this paper, we investigate the inverse problem on determining the spatial component of the source term in a hyperbolic equation with time-dependent principal part. Based on a newly established Carleman estimate for general hyperbolic operators, we prove a local stability result of Hölder type in both cases of partial boundary and interior observation data. Numerically, we adopt the classical Tikhonov regularization to transform the inverse problem into an output least-squares minimization, which can be solved by the iterative thresholding algorithm. The proposed algorithm is computationally easy and efficient: the minimizer at each step has explicit solution. Abundant amounts of numerical experiments are presented to demonstrate the accuracy and efficiency of the algorithm.

math.AP↗
arXiv2015

Stability for some inverse problems for transport equations

In this article, we consider inverse problems of determining a source term and a coefficient of a first-order partial differential equation and prove conditional stability estimates with minimum boundary observation data and relaxed condition on the principal part.

math.AP↗
arXiv2015

Uniqueness in an Integral Geometry Problem and an Inverse Problem for the Kinetic Equation

In this paper, we discuss the uniqueness in an integral geometry problem in a strongly convex domain. Our problem is related to the problem of finding a Riemannian metric by the distances between all pairs of the boundary points. For the proof, the problem is reduced to an inverse source problem for a kinetic equation on a Riemannian manifold and then the uniqueness theorem is proved in semi-geodesic coordinates by using the tools of Fourier analysis.

math.DG↗
arXiv2015

Strong maximum principle for fractional diffusion equations and an application to an inverse source problem

The strong maximum principle is a remarkable characterization of parabolic equations, which is expected to be partly inherited by fractional diffusion equations. Based on the corresponding weak maximum principle, in this paper we establish a strong maximum principle for time-fractional diffusion equations with Caputo derivatives, which is slightly weaker than that for the parabolic case. As a direct application, we give a uniqueness result for a related inverse source problem on the determination of the temporal component of the inhomogeneous term.

math.AP↗
arXiv2015

Asymptotic behavior of solutions to space-time fractional diffusion equations

This article discusses the analyticity and the long-time asymptotic behavior of solutions to space-time fractional diffusion equations in $\mathbb{R}^d$. By a Laplace transform argument, we prove that the decay rate of the solution as $t\to\infty$ is dominated by the order of the time-fractional derivative. We consider the decay rate also in a bounded domain.

math.AP↗
arXiv2014

Operator theoretic approach to the Caputo derivative and the fractional diffusion equations

The Caputo time-derivative is usually defined pointwise for well-behaved functions, say, for continuously differentiable functions. Accordingly, in the theory of the partial fractional differential equations with the Caputo derivatives, the functional spaces where the solutions are looked for are often the spaces of the smooth functions that are too narrow. In this paper, we introduce a suitable definition of the Caputo derivative in the fractional Sobolev spaces and investigate it from the operator theoretic viewpoint. In particular, some important equivalences of the norms related to the fractional integration and differentiation operators in the fractional Sobolev spaces are given. These results are then applied for proving the maximal regularity of the solutions to some initial-boundary value problems for the time-fractional diffusion equation with the Caputo derivative in the fractional Sobolev spaces.

math.AP↗
arXiv2014

Inverse problem on a tree-shaped network

In this article, we prove a uniqueness result for a coefficient inverse problems regarding a wave, a heat or a Schrödinger equation set on a tree-shaped network, as well as the corresponding stability result of the inverse problem for the wave equation. The objective is the determination of the potential on each edge of the network from the additional measurement of the solution at all but one external end-points. Our idea for proving the uniqueness is to use a traditional approach in coefficient inverse problem by Carleman estimate. Afterwards, using an observability estimate on the whole network, we apply a compactness-uniqueness argument and prove the stability for the wave inverse problem.

math.AP↗
arXiv2014

Initial-boundary value problems for multi-term time-fractional diffusion equations with positive constant coefficients

In this paper, we investigate the well-posedness and the long-time asymptotic behavior for the initial-boundary value problem for multi-term time-fractional diffusion equations, where the time differentiation consists of a finite summation of Caputo derivatives with decreasing orders in (0,1) and positive constant coefficients. By exploiting several important properties of multinomial Mittag-Leffler functions, various estimates follow from the explicit solutions in form of these special functions. Then the uniqueness and continuous dependency upon initial value and source term are established, from which the continuous dependence of solution of Lipschitz type with respect to various coefficients is also verified. Finally, by a Laplace transform argument, it turns out that the decay rate of the solution as time tends to infinity is dominated by the minimum order of the time-fractional derivatives.

math.AP↗
arXiv2014

Uniqueness for inverse problems of determining orders of multi-term time-fractional derivatives of diffusion equation

This article proves the uniqueness for two kinds of inverse problems of identifying fractional orders in diffusion equations with multiple time-fractional derivatives by pointwise observation. By means of eigenfunction expansion and Laplace transform, we reduce the uniqueness for our inverse problems to the uniqueness of expansions of some special function and complete the proof.

math.AP↗
arXiv2013

Initial-boundary value problems for linear diffusion equation with multiple time-fractional derivatives

In this paper, we discuss initial-boundary value problems for linear diffusion equation with multiple time-fractional derivatives. By means of the Mittag-Leffler function and the eigenfunction expansion, we reduce the problem to an integral equation for a solution, and we apply the fixed-point theorem to prove the unique existence and the Hölder regularity of solution. For the case of the homogeneous equation, the solution can be analytically extended to a sector $\{z\in\mathbb{C};z\neq0,|\arg z|<\fracπ{2}\}$. In the case where all the coefficients of the time-fractional derivatives are positive constants, by the Laplace transform and the analyticity, we can prove that if a function satisfies the fractional diffusion equation and the homogeneous Neumann boundary condition on arbitrary subboundary as well as the homogeneous Dirichlet boundary condition on the whole boundary, then it vanishes identically.

math.AP↗