Variations on the theme of the Trotter-Kato theorem for homogenization of periodic hyperbolic systems
In $L_2(\mathbb{R}^d;\mathbb{C}^n)$, we consider a matrix elliptic second order differential operator $B_\varepsilon >0$. Coefficients of the operator $B_\varepsilon$ are periodic with respect to some lattice in $\mathbb{R}^d$ and depend on $\mathbf{x}/\varepsilon$. We study the quantitative homogenization for the solutions of the hyperbolic system $\partial _t^2\mathbf{u}_\varepsilon =-B_\varepsilon\mathbf{u}_\varepsilon$. In operator terms, we are interested in approximations of the operators $\cos (tB_\varepsilon ^{1/2})$ and $B_\varepsilon ^{-1/2}\sin (tB_\varepsilon ^{1/2})$ in suitable operator norms. Approximations for the resolvent $B_\varepsilon ^{-1}$ have been already obtained by T.~A.~Suslina. So, we rewrite hyperbolic equation as a system for the vector with components $\mathbf{u}_\varepsilon $ and $\partial _t\mathbf{u}_\varepsilon$, and consider the corresponding unitary group. For this group, we adapt the proof of the Trotter-Kato theorem by introduction of some correction term and derive hyperbolic results from elliptic ones.