Semiclassical Asymptotics 1

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### Preliminaries

#### Introduction

We are interested in high-frequency asymptotics of wave equation (and similar) $$u_{tt}-\nabla \cdot (c^2(x)\nabla u)=0 \label{eq-5.1.1}$$ and semiclassical asymptotics to SchrÃ¶dinger equation (and similar) $$-i\hbar u_t -\frac{\hbar^2}{2m} \Delta u + V(x)u=0. \label{eq-5.1.2}$$ By high-frequency solutions we mean solutions to (\ref{eq-5.1.1}) $u=e^{ik \phi (x,t)}A(x,t,k^{1})$ with $k\gg 1$ and by semiclassical asymptotics we mean solutions to (\ref{eq-5.1.2}) with $\hbar \ll 1$, $m\asymp 1$. In fact if $m \ll 1$ we divide to (\ref{eq-5.1.2}) by $m$ we get $$-ih u_t -\frac{h^2}{2} \Delta u + \frac{1}{m}V(x)u=0. \label{eq-5.1.3}$$ with $h= \hbar/2m$ and usually $\frac{1}{m}V(x)\asymp 1$ and we interested in asymptotics when $h\ll 1$. So, we consider (\ref{eq-5.1.3}). Further, (\ref{eq-5.1.1}) after division by $k^2$ and introducing $h=k^{-1}$ becomes of the similar form \begin{equation*} h^2u_{tt}-h\nabla \cdot (c^2(x)h\nabla u)=0. \end{equation*} We are looking for solutions to equation $$P(x, ih\nabla , h)u=0 \label{eq-5.1.4}$$ where now we include $t=x_0$ in $x$ and $u=e^{ih^{-1}S(x)} A(x,h^{-1})$.

Here $$P:= \sum_{\alpha:|\alpha|\le m, l} a_{\alpha,l} (-ih\nabla )^\alpha h^l \label{eq-5.1.5}$$ where $\alpha=(\alpha_0,\ldots ,\alpha_d)$ is multinidex, $|\alpha|=\alpha_0+\ldots+\alpha_d$, $\alpha!=\alpha_0!\cdots \alpha_d!$ and $p^\alpha= p_0^{\alpha_0}\cdots p_d^{\alpha_d}$.

Remark 1 in Quantum Mechanics $-ih\nabla$ is a momentum operator.

#### Action on exponent

Theorem 1 \begin{multline} e^{-ih^{-1}S(x)} P \bigl(e^{ih^{-1}S(x)} A(x)\bigr)= P_0 (x,\nabla S(x)) A +\\ h \bigl(-i\sum_j P_0^{(j)} (x, \nabla S(x))\partial_j +Q(x)\bigr)A (x)+\ldots \label{eq-5.1.6} \end{multline} where dots denote terms with higher powers of $h$ and $$Q(x)=-\frac{i}{2}\sum _{j,k} P_0^{(jk)}(x,\nabla S(x)) S_{x_jx_k} (x) +P_1(x,\nabla S(x)) \label{eq-5.1.7}$$ and we use notations $$P_l(x,p):= \sum_{\alpha:|\alpha|\le m} a_{\alpha,l} (x)p^\alpha . \label{eq-5.1.8}$$ $P^{(j)}(x,p)=\partial_{p_j} P(x,p)$, $P_{(k)}(x,p)=\partial_{x_k} P(x,p)$ etc.