Equivalence of Operator and Fourier Definitions of the Semiclassical Wavefront Set (Theorem # 7313)
Theorem
Let $U \subset \mathbb{R}^n$ be open, let $h_0>0$, and let $\mathcal{L}^n$ denote $n$-dimensional Lebesgue measure on $\mathbb{R}^n$. Let $(u_h)_{0<h\le h_0}$ be a semiclassically tempered family of distributions on $U$, meaning that for every compact set $K\Subset U$ there exist integers $m,L\geq 0$ and a constant $C_K>0$ such that
\begin{align*}
|u_h(\varphi)|\leq C_K h^{-L}\sum_{|\alpha|\leq m}\sup_{x\in K}|\partial_x^\alpha\varphi(x)|
\end{align*}
for every $\varphi\in C_c^\infty(U)$ with $\operatorname{supp}\varphi\subset K$ and every $0<h\leq h_0$. Let $(x_0,\xi_0)\in T^*U$ with $\xi_0\in\mathbb{R}^n$. For $\phi \in C_c^\infty(U)$, define the semiclassical Fourier transform of $\phi u_h$ by
\begin{align*}
\mathcal{F}_h(\phi u_h)(\xi) = (\phi u_h)\bigl(x \mapsto e^{-i x\cdot \xi/h}\bigr).
\end{align*}
Let $\operatorname{Op}_h(a)$ denote a properly supported semiclassical quantization in the standard properly supported semiclassical pseudodifferential calculus on $U$ for symbols $a\in C_c^\infty(T^*U)$, whose action on compactly supported functions is given by
\begin{align*}
\operatorname{Op}_h(a)f(x)=(2\pi h)^{-n}\int_{\mathbb{R}^n}e^{ix\cdot\xi/h}a(x,\xi)\mathcal{F}_h f(\xi)\,d\mathcal{L}^n(\xi),
\end{align*}
and whose extension to distributions is defined by its properly supported Schwartz kernel. Assume this calculus has asymptotic composition, microlocal elliptic parametrices for symbols bounded away from zero on compact subsets of $T^*U$, proper-support continuity on local distribution and smooth seminorms, and residual operators whose properly supported Schwartz kernels are $O(h^M)$ with all derivatives for every $M\geq0$. Say that $(x_0,\xi_0)\notin \operatorname{WF}_h(u_h)$ in the operator-theoretic finite-fiber sense if there exists $a\in C_c^\infty(T^*U)$ with $a(x_0,\xi_0)\neq 0$ such that $\operatorname{Op}_h(a)u_h=O(h^\infty)$ in $C^\infty_{\mathrm{loc}}(U)$. Then $(x_0,\xi_0)\notin \operatorname{WF}_h(u_h)$ in the operator-theoretic finite-fiber sense if and only if there exist $\chi\in C_c^\infty(U)$ with $\chi(x_0)\neq 0$ and an open neighbourhood $V\subset\mathbb{R}^n$ of $\xi_0$ such that, for every $N\geq 0$ and every $M\geq 0$, there exist constants $C_{N,M}>0$ and $h_{N,M}\in(0,h_0]$ satisfying
\begin{align*}
|\mathcal{F}_h(\chi u_h)(\xi)| \leq C_{N,M} h^M \langle \xi\rangle^{-N}
\end{align*}
for all $\xi\in V$ and all $0<h\leq h_{N,M}$.
Knowledge Status
Analysis
Discussion
No discussion available for this theorem.
Proof
[proofplan]
We prove the equivalence in the two directions. Starting from the operator definition, an elliptic annihilator near $(x_0,\xi_0)$ admits a semiclassical parametrix; after shrinking the spatial and frequency neighbourhoods, this expresses the localized distribution $\chi u_h$ microlocally over $V$ as a pseudodifferential image of an $O(h^\infty)$ family plus a residual term, hence its semiclassical Fourier transform is rapidly small. Conversely, the Fourier decay assumption allows us to choose a compactly supported symbol elliptic at $(x_0,\xi_0)$ and supported in the frequency region where the Fourier transform is rapidly small. The resulting operator kills $\chi u_h$ by direct Fourier inversion, while its action on $(1-\chi)u_h$ is residual because the spatial supports are separated.
[/proofplan]
[step:Pass from an elliptic annihilator to microlocal Fourier decay]
Assume first that $(x_0,\xi_0)\notin \operatorname{WF}_h(u_h)$ in the operator-theoretic sense. By definition, there exist a symbol $a\in C_c^\infty(T^*U)$ elliptic at $(x_0,\xi_0)$ and the corresponding semiclassical operator $A_h=\operatorname{Op}_h(a)$ such that
\begin{align*}
A_h u_h = O(h^\infty)
\end{align*}
in $C^\infty_{\mathrm{loc}}(U)$.
Choose open sets $U_1\Subset U$ and $V\subset\mathbb{R}^n$ with $x_0\in U_1$ and $\xi_0\in V$ such that $a$ is elliptic on $U_1\times V$. Choose $\chi:U\to\mathbb{R}$ with $\chi\in C_c^\infty(U_1)$ and $\chi(x_0)\neq 0$.
Choose an open neighbourhood $V_0\Subset V$ of $\xi_0$. We use the semiclassical elliptic parametrix theorem and composition calculus in the following precise form. Because $a$ is elliptic on a neighbourhood of $\operatorname{supp}\chi\times\overline{V_0}$, there is a properly supported operator $B_h=\operatorname{Op}_h(b)$, with $b\in C_c^\infty(T^*U)$, such that
\begin{align*}
B_hA_h=\operatorname{Op}_h(\chi)+C_h+S_h
\end{align*}
on distributions over $U$, where $S_h$ is residual and $C_h=\operatorname{Op}_h(c)$ has symbol $c\in C_c^\infty(T^*U)$ whose frequency support is disjoint from an open neighbourhood of $V_0$. More concretely, $b$ is chosen equal to $\chi(x)/a(x,\xi)$ on a neighbourhood of $\operatorname{supp}\chi\times\overline{V_0}$, so the full symbol of $\operatorname{Op}_h(\chi)-B_hA_h$ vanishes to all orders in $h$ over $\operatorname{supp}\chi\times V_0$; the remaining nonresidual symbol may therefore be cut off in the complementary frequency region. The hypotheses of this parametrix construction hold here because $a\in C_c^\infty(T^*U)$), $a$ is bounded away from zero on a neighbourhood of $\operatorname{supp}\chi\times\overline{V_0}$, $\operatorname{supp}\chi\Subset U_1$, and $\overline{V_0}\subset V$.
Applying this identity to $u_h$ gives
\begin{align*}
\chi u_h=B_hA_hu_h-C_hu_h-S_hu_h.
\end{align*}
The term $C_hu_h$ has rapidly small semiclassical Fourier transform on $V_0$. Indeed, let $K_y\Subset U$ be the compact $y$-support supplied by proper support over a fixed compact output set containing $\operatorname{supp}\chi$. Semiclassical temperedness gives integers $m,L\geq0$ and a constant $C_K>0$ controlling the action of $u_h$ on test functions supported in $K_y$. In the Fourier representation of $C_h$ the output frequency $\xi\in V_0$ is separated by a positive distance from the compact frequency support of $c$. Repeated integration by parts in the spatial variables using the nonstationary phase factor $e^{i x\cdot(\eta-\xi)/h}$ therefore gives, after choosing more than $M+L+n+N+m$ integrations by parts,
\begin{align*}
|\mathcal{F}_h(C_hu_h)(\xi)|\leq C_{N,M}h^M\langle\xi\rangle^{-N}
\end{align*}
for all $\xi\in V_0$ and all sufficiently small $h$. The constant $C_{N,M}$ depends only on the separation of $V_0$ from the frequency support of $c$, finitely many seminorms of $c$, and the above local distribution seminorm bound for $(u_h)$.
Since $A_hu_h=O(h^\infty)$ in $C^\infty_{\mathrm{loc}}(U)$ and $B_h$ is properly supported with compact spatial support over $U_1$, the standard continuity of properly supported semiclassical pseudodifferential operators on local smooth seminorms gives $B_hA_hu_h=O(h^\infty)$ in $C^\infty_{\mathrm{loc}}(U)$. Since $(u_h)$ is semiclassically tempered and $S_h$ is residual, the residual mapping property gives
\begin{align*}
S_hu_h = O(h^\infty)
\end{align*}
in $C^\infty(U)$ after multiplication by any compactly supported cutoff. The semiclassical Fourier transform of a compactly supported $O(h^\infty)$ smooth family is $O(h^\infty)\langle\xi\rangle^{-N}$ for every $N$, by repeated integration by parts in the spatial variable. Taking semiclassical Fourier transforms in the preceding decomposition therefore gives, for every $N\geq 0$ and $M\geq 0$,
\begin{align*}
|\mathcal{F}_h(\chi u_h)(\xi)| \leq C_{N,M}h^M\langle\xi\rangle^{-N}
\end{align*}
uniformly for $\xi\in V_0$. Replacing $V$ by $V_0$ gives the desired Fourier decay condition.
[guided]
The operator definition says that absence from $\operatorname{WF}_h(u_h)$ is detected by an elliptic pseudodifferential operator that kills $u_h$ to infinite order in $h$. Thus we begin with a compactly supported symbol $a\in C_c^\infty(T^*U)$, elliptic at $(x_0,\xi_0)$, and set $A_h=\operatorname{Op}_h(a)$, with
\begin{align*}
A_hu_h = O(h^\infty)
\end{align*}
locally smoothly.
Ellipticity is an open condition. Therefore we may choose an open spatial neighbourhood $U_1\Subset U$ of $x_0$ and an open frequency neighbourhood $V\subset\mathbb{R}^n$ of $\xi_0$ such that $a$ remains elliptic on $U_1\times V$. We then choose $\chi:U\to\mathbb{R}$ with $\chi\in C_c^\infty(U_1)$ and $\chi(x_0)\neq 0$.
The role of ellipticity is that it permits microlocal division by $a$, but this division must be performed at the level of the full semiclassical symbol, not only at the principal-symbol level. Choose an open neighbourhood $V_0\Subset V$ of $\xi_0$. The semiclassical elliptic parametrix theorem in the properly supported calculus specified in the theorem statement gives a properly supported operator $B_h=\operatorname{Op}_h(b)$ with a compactly supported classical symbol $b\sim\sum_{j=0}^{\infty}h^j b_j$. The leading term satisfies $b_0=\chi(x)/a(x,\xi)$ on a neighbourhood of $\operatorname{supp}\chi\times\overline{V_0}$, and the lower-order terms $b_j$ are chosen recursively by the composition formula so that the full symbol of $\operatorname{Op}_h(\chi)-B_hA_h$ vanishes to every order in $h$ on $\operatorname{supp}\chi\times V_0$. Cutting the remaining full symbol away from $V_0$ gives
\begin{align*}
B_hA_h=\operatorname{Op}_h(\chi)+C_h+S_h,
\end{align*}
where $S_h$ is residual and $C_h=\operatorname{Op}_h(c)$ has compactly supported symbol $c$ whose frequency support is disjoint from an open neighbourhood of $V_0$.
We verify the hypotheses carefully. The symbol $a$ is compactly supported in $T^*U$. Since $a$ is elliptic on $U_1\times V$, it is bounded away from zero on a neighbourhood of the compact set $\operatorname{supp}\chi\times\overline{V_0}$. This is exactly the region on which division by $a$ is performed. Proper support is part of the quantization convention in the theorem statement, so all operators act on distributions on $U$ by compactly supported kernel slices.
Applying the identity to $u_h$ gives
\begin{align*}
\chi u_h=B_hA_hu_h-C_hu_h-S_hu_h.
\end{align*}
The first term is rapidly negligible because $A_hu_h=O(h^\infty)$ in local smooth seminorms and properly supported pseudodifferential operators preserve such $O(h^\infty)$ bounds locally. The residual term is rapidly negligible because residual properly supported kernels are $O(h^\infty)$ with all derivatives, and semiclassical temperedness of $(u_h)$ absorbs the fixed polynomial loss in $h^{-1}$ coming from distribution seminorms.
It remains to explain why $C_hu_h$ has Fourier decay on $V_0$. The output frequency $\xi\in V_0$ is separated from the frequency support of $c$. In the Fourier representation of $\mathcal{F}_h(C_hu_h)(\xi)$, this separation produces a nonstationary phase factor with derivative proportional to $\eta-\xi$, where $\eta$ is the internal frequency variable of $C_h$. Since $|\eta-\xi|$ is bounded below on the support of the amplitude, repeated integration by parts in the spatial variables gains a factor of $h$ at each integration. Proper support reduces the distributional pairing to a fixed compact set $K_y\Subset U$ in the input variable. Semiclassical temperedness on $K_y$ gives a finite derivative order $m$ and a polynomial loss $h^{-L}$. Choosing the number of integrations by parts larger than this loss, and also larger than the requested powers $M$ and $N$, gives
\begin{align*}
|\mathcal{F}_h(C_hu_h)(\xi)|\leq C_{N,M}h^M\langle\xi\rangle^{-N}
\end{align*}
uniformly for $\xi\in V_0$. The constant comes from the separation from the frequency support of $c$, finitely many seminorms of $c$, and the local distribution seminorm bound for $(u_h)$. Combining the estimates for the three terms gives the same bound for $\mathcal{F}_h(\chi u_h)$ on $V_0$. Since $V_0$ is still an open neighbourhood of $\xi_0$, this proves the required Fourier decay direction.
[/guided]
[/step]
[step:Choose an elliptic symbol supported inside the Fourier decay region]
Assume conversely that there exist $\chi\in C_c^\infty(U)$ with $\chi(x_0)\neq 0$ and an open neighbourhood $V$ of $\xi_0$ such that the stated Fourier decay estimate holds.
Since $\chi(x_0)\neq 0$, choose open sets $W_0\Subset W_1\Subset U$ with $x_0\in W_0$, $\chi$ nowhere zero on $W_1$, and $\overline{W_1}\subset U$. Choose $q:U\to\mathbb{C}$ with $q\in C_c^\infty(W_1)$ and $q\chi=1$ on a neighbourhood of $\overline{W_0}$. Choose an open neighbourhood $V_0\Subset V$ of $\xi_0$. The family $q\chi u_h$ has the same rapid Fourier decay on $V_0$. Indeed, write $q\chi u_h=q(\chi u_h)$. Multiplication by $q\in C_c^\infty(U)$ gives the semiclassical convolution identity
\begin{align*}
\mathcal{F}_h(q\chi u_h)(\xi)=(2\pi h)^{-n}\int_{\mathbb{R}^n}\mathcal{F}_h q(\xi-\eta)\mathcal{F}_h(\chi u_h)(\eta)\,d\mathcal{L}^n(\eta)
\end{align*}
for $\xi\in\mathbb{R}^n$, interpreted by the compact support of $q\chi u_h$. By repeated integration by parts in the defining integral for $\mathcal{F}_h q$, for every $R\geq0$ and $J\geq0$ there is $C_{R,J}>0$ such that
\begin{align*}
|\mathcal{F}_h q(\zeta)|\leq C_{R,J}h^J\langle\zeta\rangle^{-R}
\end{align*}
for all $\zeta\in\mathbb{R}^n$. Split the $\eta$-integral into $\eta\in V$ and $\eta\notin V$. On $V$, the assumed estimate for $\mathcal{F}_h(\chi u_h)$ gives arbitrary powers of $h$ and $\langle\eta\rangle^{-N}$, and the rapid decay of $\mathcal{F}_h q(\xi-\eta)$ makes the convolution finite uniformly for $\xi\in V_0$. On $\mathbb{R}^n\setminus V$, the positive distance $\operatorname{dist}(V_0,\mathbb{R}^n\setminus V)>0$ allows the same integration-by-parts estimate for $\mathcal{F}_h q(\xi-\eta)$ to supply an arbitrary power of $h$; the remaining polynomial growth of $\mathcal{F}_h(\chi u_h)$ is bounded by the semiclassical temperedness of $(u_h)$ applied to the compactly supported distribution $\chi u_h$. Choosing $R$ and $J$ larger than the requested decay order and the temperedness loss proves the same estimate for $\mathcal{F}_h(q\chi u_h)$ on $V_0$.
Choose $\rho:U\to[0,1]$ with $\rho\in C_c^\infty(W_0)$ and $\rho(x_0)\neq 0$. Then $q\chi=1$ on a neighbourhood of $\operatorname{supp}\rho$. Choose $\psi:\mathbb{R}^n\to[0,1]$ with $\psi\in C_c^\infty(V_0)$ and $\psi(\xi_0)\neq 0$. Define
\begin{align*}
a:T^*U\to\mathbb{C},\qquad a(x,\xi)=\rho(x)\psi(\xi).
\end{align*}
Then $a\in C_c^\infty(T^*U)$ and $a$ is elliptic at $(x_0,\xi_0)$ because $\rho(x_0)\psi(\xi_0)\neq 0$. Let $A_h=\operatorname{Op}_h(a)$.
[guided]
The point of this step is to build an operator whose symbol is nonzero at $(x_0,\xi_0)$ but whose frequency support lies where the Fourier transform is assumed to be rapidly small. Since $\chi(x_0)\neq 0$, continuity gives a neighbourhood $W_1$ of $x_0$ on which $\chi$ is nowhere zero. We choose $W_0\Subset W_1\Subset U$ with $x_0\in W_0$ and then choose $q\in C_c^\infty(W_1)$ so that $q\chi=1$ on a neighbourhood of $\overline{W_0}$.
We must justify that replacing $\chi u_h$ by $q\chi u_h$ does not destroy the Fourier decay. The multiplication is by the fixed compactly supported smooth function $q$, and in frequency space it gives
\begin{align*}
\mathcal{F}_h(q\chi u_h)(\xi)=(2\pi h)^{-n}\int_{\mathbb{R}^n}\mathcal{F}_h q(\xi-\eta)\mathcal{F}_h(\chi u_h)(\eta)\,d\mathcal{L}^n(\eta).
\end{align*}
The factor $\mathcal{F}_h q$ is rapidly decaying with arbitrary powers of $h$: repeated integration by parts in the compact $x$-support of $q$ gives
\begin{align*}
|\mathcal{F}_h q(\zeta)|\leq C_{R,J}h^J\langle\zeta\rangle^{-R}
\end{align*}
for all $R,J\geq0$. For $\eta\in V$, the assumed rapid estimate controls $\mathcal{F}_h(\chi u_h)(\eta)$. For $\eta\notin V$ and $\xi\in V_0$, the positive distance from $V_0$ to $\mathbb{R}^n\setminus V$ lets us choose $J$ large enough in the estimate for $\mathcal{F}_h q(\xi-\eta)$ to dominate the polynomial $h^{-1}$ loss allowed by semiclassical temperedness of $\chi u_h$. Taking $R$ large makes the convolution integrable with the desired $\langle\xi\rangle^{-N}$ weight. Thus $\mathcal{F}_h(q\chi u_h)$ satisfies the same $O(h^\infty)\langle\xi\rangle^{-N}$ estimate on $V_0$.
Now choose $\rho\in C_c^\infty(W_0)$ with $\rho(x_0)\neq 0$ and choose $\psi\in C_c^\infty(V_0)$ with $\psi(\xi_0)\neq 0$. Define the symbol as the map
\begin{align*}
a:T^*U&\to\mathbb{C}
\end{align*}
by $a(x,\xi)=\rho(x)\psi(\xi)$. This symbol is compactly supported, and $a(x_0,\xi_0)=\rho(x_0)\psi(\xi_0)\neq 0$, so it is elliptic at $(x_0,\xi_0)$.
[/guided]
[/step]
[step:Show that the operator kills the localized piece]
We first prove that $A_h(q\chi u_h)=O(h^\infty)$ in $C^\infty(U)$. Since $q\chi u_h$ has compact support, the Fourier representation of the left quantization gives
\begin{align*}
A_h(q\chi u_h)(x) = (2\pi h)^{-n}\int_{\mathbb{R}^n} e^{i x\cdot\xi/h} a(x,\xi)\mathcal{F}_h(q\chi u_h)(\xi)\,d\mathcal{L}^n(\xi).
\end{align*}
Because $\operatorname{supp}\psi\subset V_0$, the integral is taken over a compact subset of $V_0$. For every multi-index $\alpha$, differentiating in $x$ gives a finite sum of terms of the form
\begin{align*}
(2\pi h)^{-n}h^{-|\beta|}\int_{\operatorname{supp}\psi} e^{i x\cdot\xi/h} c_{\alpha,\beta}(x,\xi)\mathcal{F}_h(q\chi u_h)(\xi)\,d\mathcal{L}^n(\xi),
\end{align*}
where $c_{\alpha,\beta}\in C_c^\infty(U\times V_0)$ is obtained from derivatives of $a$ and multiplication by monomials in $\xi$. The set $\operatorname{supp}\psi$ is compact, hence has finite $\mathcal{L}^n$-measure, and every monomial in $\xi$ that appears in $c_{\alpha,\beta}$ is bounded on $\operatorname{supp}\psi$. The Fourier decay estimate for $q\chi u_h$, with $M$ replaced by $M+n+|\beta|+1$, therefore bounds each such term by $C_{\alpha,M}h^M$ uniformly for $x$ in compact subsets of $U$; here $C_{\alpha,M}$ is obtained explicitly from the relevant Fourier decay constant, the finite list of seminorms of $a$ appearing in $c_{\alpha,\beta}$, the supremum of the finitely many $\xi$-monomials on $\operatorname{supp}\psi$, and $\mathcal{L}^n(\operatorname{supp}\psi)<\infty$. Therefore
\begin{align*}
A_h(q\chi u_h)=O(h^\infty)
\end{align*}
in $C^\infty(U)$.
[guided]
We estimate the operator directly from its Fourier representation. Since $q\chi u_h$ is compactly supported, the formula for left quantization is valid:
\begin{align*}
A_h(q\chi u_h)(x) = (2\pi h)^{-n}\int_{\mathbb{R}^n} e^{i x\cdot\xi/h} a(x,\xi)\mathcal{F}_h(q\chi u_h)(\xi)\,d\mathcal{L}^n(\xi).
\end{align*}
Because $a(x,\xi)=\rho(x)\psi(\xi)$ and $\operatorname{supp}\psi\subset V_0$, the integral only sees frequencies where $\mathcal{F}_h(q\chi u_h)$ is rapidly small. If $\alpha$ is a multi-index, differentiating in $x$ differentiates the amplitude and may also differentiate $e^{i x\cdot\xi/h}$; each derivative landing on the exponential contributes a factor $h^{-1}\xi_j$. Therefore every differentiated term is a finite sum of integrals of the form
\begin{align*}
(2\pi h)^{-n}h^{- |\beta|}\int_{\operatorname{supp}\psi} e^{i x\cdot\xi/h} c_{\alpha,\beta}(x,\xi)\mathcal{F}_h(q\chi u_h)(\xi)\,d\mathcal{L}^n(\xi),
\end{align*}
where $c_{\alpha,\beta}\in C_c^\infty(U\times V_0)$ is built from derivatives of $a$ and monomials in $\xi$.
The constants are finite for concrete reasons: $\operatorname{supp}\psi$ is compact and hence has finite $\mathcal{L}^n$-measure, the finitely many $\xi$-monomials are bounded on $\operatorname{supp}\psi$, and only finitely many seminorms of $a$ occur for a fixed $\alpha$. Taking the Fourier decay estimate for $q\chi u_h$ with the exponent $M+n+|\beta|+1$ absorbs the prefactor $(2\pi h)^{-n}h^{-|\beta|}$. Thus each differentiated term is $O(h^M)$ uniformly on compact subsets of $U$, and consequently
\begin{align*}
A_h(q\chi u_h)=O(h^\infty)
\end{align*}
in $C^\infty(U)$.
[/guided]
[/step]
[step:Remove the auxiliary spatial cutoff by nonstationary phase]
It remains to control $A_h((1-q\chi)u_h)$. The factor $\rho(x)$ is contained in the symbol $a(x,\xi)=\rho(x)\psi(\xi)$, so the Schwartz kernel only involves $x\in\operatorname{supp}\rho$. Since $q\chi=1$ on a neighbourhood of $\operatorname{supp}\rho$, choose $\theta:U\to[0,1]$ with $\theta\in C_c^\infty(U)$, $\theta=1$ on a neighbourhood of $\operatorname{supp}\rho$, and $q\chi=1$ on a neighbourhood of $\operatorname{supp}\theta$. Then
\begin{align*}
A_h((1-q\chi)u_h)=A_h((1-q\chi)\theta u_h)+A_h((1-q\chi)(1-\theta)u_h).
\end{align*}
The first term is identically zero because $(1-q\chi)\theta=0$.
For the second term, the spatial supports of $\rho$ and $(1-q\chi)(1-\theta)$ are separated. Thus there exists $\delta>0$ such that
\begin{align*}
|x-y|\geq \delta
\end{align*}
whenever $x\in\operatorname{supp}\rho$ and $y\in\operatorname{supp}(1-\theta)$.
The Schwartz kernel of $A_h(1-q\chi)(1-\theta)$ is
\begin{align*}
K_h(x,y)=(2\pi h)^{-n}\int_{\mathbb{R}^n}e^{i(x-y)\cdot\xi/h}a(x,\xi)(1-q(y)\chi(y))(1-\theta(y))\,d\mathcal{L}^n(\xi).
\end{align*}
Because the quantization in the theorem statement is properly supported, for every compact output set $K_x\Subset U$ there is a compact set $K_y\Subset U$ such that the Schwartz kernel of $A_h$ over $K_x\times U$ is supported in $K_x\times K_y$. Hence, when estimating $A_h((1-q\chi)(1-\theta)u_h)$ on $K_x$, the distribution $u_h$ is applied only to compactly supported test functions in the $y$ variable.
For every compact set $K_x\Subset U$, every pair of multi-indices $\alpha$ and $\gamma$, and every $M\geq0$, repeated integration by parts in $\xi$ with
\begin{align*}
L_{x,y,\xi}=\frac{h}{i|x-y|^2}\sum_{j=1}^n (x_j-y_j)\partial_{\xi_j}
\end{align*}
on the compact set $(K_x\cap\operatorname{supp}\rho)\times K_y$ gives
\begin{align*}
\sup_{x\in K_x,\,y\in K_y}|\partial_x^\alpha\partial_y^\gamma K_h(x,y)|\leq C_{\alpha,\gamma,K_x,M}h^M.
\end{align*}
Here the constants are finite because $|x-y|\geq\delta$ on the support of the amplitude, the $\xi$-support of $a$ is compact, and only finitely many derivatives of $a$, $q\chi$, and $\theta$ occur. Since $(u_h)$ is semiclassically tempered as a distribution on $U$, its action on compactly supported test functions in $K_y$ grows at most polynomially in $h^{-1}$ in local distribution seminorms. Choosing the number of integrations by parts larger than this polynomial loss gives
\begin{align*}
A_h((1-q\chi)(1-\theta)u_h)=O(h^\infty)
\end{align*}
in $C^\infty_{\mathrm{loc}}(U)$, and therefore also
\begin{align*}
A_h((1-q\chi)u_h)=O(h^\infty)
\end{align*}
in $C^\infty_{\mathrm{loc}}(U)$.
Combining this with the previous step gives
\begin{align*}
A_hu_h=A_h(q\chi u_h)+A_h((1-q\chi)u_h)=O(h^\infty)
\end{align*}
in $C^\infty_{\mathrm{loc}}(U)$.
[guided]
We still have to remove the auxiliary factor $q\chi$. Since $q\chi=1$ near $\operatorname{supp}\rho$, the symbol of $A_h$ only observes $x$ in a region where $1-q\chi$ vanishes. Choose $\theta\in C_c^\infty(U)$ with $\theta=1$ near $\operatorname{supp}\rho$ and with $q\chi=1$ near $\operatorname{supp}\theta$. Then
\begin{align*}
A_h((1-q\chi)u_h)=A_h((1-q\chi)\theta u_h)+A_h((1-q\chi)(1-\theta)u_h).
\end{align*}
The first term is zero because $(1-q\chi)\theta=0$.
For the second term, the $x$-support of the operator is contained in $\operatorname{supp}\rho$, while the $y$-support of $(1-\theta)$ is separated from $\operatorname{supp}\rho$. Thus there is $\delta>0$ such that $|x-y|\geq\delta$ on the relevant support. The proper support assumption in the theorem statement is essential here: on each compact output set $K_x\Subset U$, the kernel only pairs $u_h$ with test functions supported in some compact set $K_y\Subset U$. Therefore the distribution $u_h$ is being applied legitimately.
On $K_x\times K_y$, repeated integration by parts in $\xi$ with
\begin{align*}
L_{x,y,\xi}=\frac{h}{i|x-y|^2}\sum_{j=1}^n (x_j-y_j)\partial_{\xi_j}
\end{align*}
gains one factor of $h$ each time because the phase derivative is bounded below by $\delta$. The compact $\xi$-support of $a$ and the finite number of derivatives of $a$, $q\chi$, and $\theta$ control the constants. After enough integrations by parts, the resulting $h^M$ gain dominates the polynomial $h^{-1}$ growth allowed by semiclassical temperedness of $(u_h)$. Hence
\begin{align*}
A_h((1-q\chi)u_h)=O(h^\infty)
\end{align*}
in $C^\infty_{\mathrm{loc}}(U)$. Combining this estimate with $A_h(q\chi u_h)=O(h^\infty)$ gives $A_hu_h=O(h^\infty)$ locally smoothly.
[/guided]
[/step]
[step:Conclude the equivalence with the operator definition]
The symbol $a(x,\xi)=\rho(x)\psi(\xi)$ is compactly supported and elliptic at $(x_0,\xi_0)$, and the corresponding semiclassical operator satisfies
\begin{align*}
A_hu_h=O(h^\infty)
\end{align*}
locally smoothly. By the operator-theoretic definition of the semiclassical wavefront set, this means
\begin{align*}
(x_0,\xi_0)\notin\operatorname{WF}_h(u_h).
\end{align*}
Together with the first direction, this proves the equivalence.
[/step]
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