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In this paper we try to complete the stability analysis for an abstract system of coupled hyperbolic and parabolic equations $$ \left\{ \begin{array}{lll} \ds u_{tt} + Au - A^\alpha w = 0, \\ w_t + A^\alpha u_t + A^\beta w = 0,\\ u(0) = u_0, u_t(0) = u_1, w(0) = w_0, \end{array} \right. $$ where $A$ is a self-adjoint, positive definite operator on a complex Hilbert space $H$, and $(\alpha, \beta) \in [0,1] \times [0,1]$, which is considered in \cite{Amk}, and after, in \cite{liu1}. Our contribution is to identify a fine scale of polynomial stability of the solution in the region $ S_3: = \left\{(\alpha,\beta) \in [0,1] \times [0,1]; \, \beta < 2\alpha -1 \right\}$ taking into account the presence of a singularity at zero.

This paper is on the asymptotic behavior of the elastic string equation with localized degenerate Kelvin--Voigt damping $$ u_{tt}(x,t)-[u_{x}(x,t)+b(x)u_{x,t}(x,t)]_{x}=0,\; x\in(-1,1),\; t>0,$$ where $b(x)=0$ on $x\in (-1,0]$, and $b(x)=x^\alpha>0$ on $x\in (0,1)$ for $\alpha\in(0,1)$. It is known that the optimal decay rate of solution is $t^{-2}$ in the limit case $\alpha=0$, and exponential decay rate for $\alpha\ge 1$. When $\alpha\in (0,1)$, the damping coefficient $b(x)$ is continuous, but its derivative has a singularity at the interface $x=0$. In this case, the best known decay rate is $t^{-\frac{3-\alpha}{2(1-\alpha)}}$. Although this rate is consistent with the exponential one at $\alpha=1$, it failed to match the optimal one at $\alpha=0$. In this paper, we obtain a sharper polynomial decay rate $t^{-\frac{2-\alpha}{1-\alpha}}$. More significantly, it is consistent with the optimal polynomial decay rate at $\alpha=0$ and the exponential decay rate at $\alpha = 1$.This is a big step toward the goal of obtaining eventually the optimal decay rate.

In this paper, we consider the stabilization of the generalized Rao-Nakra beam equation, which consists of four wave equations for the longitudinal displacements and the shear angle of the top and bottom layers and one Euler-Bernoulli beam equation for the transversal displacement. Dissipative mechanism are provided through viscous damping for two displacements. The location of the viscous damping are divided into two groups, characterized by whether both of the top and bottom layers are directly damped or otherwise. Each group consists of three cases. We obtain the necessary and sufficient conditions for the cases in group two to be strongly stable. Furthermore, polynomial stability of certain orders are proved. The cases in group one are left for future study

In this paper we study the stability problem of a tree of elastic strings with local Kelvin-Voigt damping on some of the edges. Under the compatibility condition of displacement and strain and continuity condition of damping coefficients at the vertices of the tree, exponential/polynomial stability are proved.

In this paper, we provide a complete regularity analysis for an abstract system of coupled hyperbolic and parabolic equations in a complex Hilbert space. We are able to decompose the unit square of the parameters into three parts where the semigroup associated with the system is analytic, of specific order Gevrey classes, and non-smoothing, respectively. Moreover, we will show that the orders of Gevrey class are sharp, under proper conditions., Comment: 30 pages, 3 figures