Kodaira–Serre $L^2$ Estimate for High Tensor Powers (Theorem # 3896)
Theorem
Let $(X,\omega)$ be a compact Kähler manifold of complex dimension $n$, and let $(L,h)$ be a positive holomorphic Hermitian line bundle over $X$. For every integer $q$ with $1 \le q \le n$, there exist an integer $m_0 \in \mathbb{N}$ and a constant $C > 0$ such that the following holds.
For every integer $m \ge m_0$ and every smooth $L^m$-valued $(0,q)$-form
\begin{align*}
f \in C^\infty\bigl(X;\Lambda^{0,q}T^*X \otimes L^m\bigr)
\end{align*}
satisfying $\bar\partial f = 0$, there exists a smooth $L^m$-valued $(0,q-1)$-form
\begin{align*}
u \in C^\infty\bigl(X;\Lambda^{0,q-1}T^*X \otimes L^m\bigr)
\end{align*}
such that
\begin{align*}
\bar\partial u &= f, &
\|u\|_{L^2(X,\omega,h^m)}^2 &\le \frac{C}{m}\|f\|_{L^2(X,\omega,h^m)}^2.
\end{align*}
Discussion
For sufficiently large tensor powers of a positive line bundle, every bar partial closed valued form of type 0,q with q at least 1 has a solution to the bar partial equation with an L2 norm estimate of order 1 over m. The theorem provides the analytic coercivity used to solve bar partial equations uniformly in high powers of a positive bundle.
Proof
[proofplan]
We convert the problem for $L^m$-valued $(0,q)$-forms into the standard Bochner--Kodaira--Nakano estimate on $(n,q)$-forms by tensoring with the inverse canonical bundle. The curvature of $K_X^{-1}\otimes L^m$ is the sum of a fixed curvature term and $m$ times the positive curvature of $L$, so for $m$ large the positive part dominates uniformly on all $(n,q)$-forms with $q\ge 1$. This gives a coercive estimate for the $\bar\partial$-Laplacian. The Hilbert-space existence theorem for $\bar\partial$ then produces the minimal solution, and elliptic regularity upgrades it to a smooth form.
[/proofplan]
[step:Replace $(0,q)$-forms with $(n,q)$-forms valued in $K_X^{-1}\otimes L^m$]
Let $K_X := \Lambda^{n,0}T^*X$ denote the canonical bundle, equipped with the Hermitian metric induced by $\omega$. Define
\begin{align*}
E_m &:= K_X^{-1}\otimes L^m
\end{align*}
with the product Hermitian metric $g_m$ induced by the dual metric on $K_X^{-1}$ and by $h^m$ on $L^m$.
There is a natural bundle isomorphism
\begin{align*}
\mathcal{I}_m:\Lambda^{0,q}T^*X\otimes L^m &\longrightarrow \Lambda^{n,q}T^*X\otimes E_m \\
\alpha &\longmapsto \alpha \otimes \operatorname{id}_{K_X},
\end{align*}
where $\operatorname{id}_{K_X}$ denotes the canonical identity endomorphism of $K_X$ under the trace-contraction identification
\begin{align*}
K_X\otimes K_X^{-1}\cong \operatorname{End}(K_X).
\end{align*}
Equivalently, this is the tensor contraction identification
\begin{align*}
\Lambda^{0,q}T^*X\otimes L^m
\cong
\Lambda^{n,q}T^*X\otimes K_X^{-1}\otimes L^m.
\end{align*}
Because the Hermitian metrics on both sides are induced from the same Kähler metric $\omega$ and the same line bundle metric $h^m$, $\mathcal{I}_m$ is an isometry for the corresponding $L^2$ norms.
The map $\mathcal{I}_m$ is holomorphic in the coefficient bundle and commutes with $\bar\partial$. Therefore, if $f$ is a smooth $L^m$-valued $(0,q)$-form with $\bar\partial f=0$, then
\begin{align*}
F_m := \mathcal{I}_m f
\end{align*}
is a smooth $E_m$-valued $(n,q)$-form with $\bar\partial F_m=0$ and
\begin{align*}
\|F_m\|_{L^2(X,\omega,g_m)}=\|f\|_{L^2(X,\omega,h^m)}.
\end{align*}
[guided]
The Bochner--Kodaira--Nakano curvature term has the strongest positivity on $(n,q)$-forms. Our theorem is stated for $(0,q)$-forms, so we first rewrite the same objects in top holomorphic degree.
Let $K_X := \Lambda^{n,0}T^*X$ be the canonical bundle. Since
\begin{align*}
\Lambda^{n,q}T^*X
=
\Lambda^{n,0}T^*X\otimes \Lambda^{0,q}T^*X
=
K_X\otimes \Lambda^{0,q}T^*X,
\end{align*}
tensoring by $K_X^{-1}$ gives the bundle identification
\begin{align*}
\Lambda^{0,q}T^*X\otimes L^m
\cong
\Lambda^{n,q}T^*X\otimes K_X^{-1}\otimes L^m.
\end{align*}
We therefore define
\begin{align*}
E_m := K_X^{-1}\otimes L^m.
\end{align*}
The isomorphism
\begin{align*}
\mathcal{I}_m:\Lambda^{0,q}T^*X\otimes L^m &\longrightarrow \Lambda^{n,q}T^*X\otimes E_m \\
\alpha &\longmapsto \alpha \otimes \operatorname{id}_{K_X}
\end{align*}
is the tensor-contraction identification obtained by viewing $\operatorname{id}_{K_X}$ as the canonical identity endomorphism under
\begin{align*}
K_X\otimes K_X^{-1}\cong \operatorname{End}(K_X).
\end{align*}
The metrics on both sides are induced from $\omega$ and $h^m$, so this identification preserves pointwise norms and hence preserves $L^2$ norms after integration against the Kähler volume measure $dV_\omega(x)$:
\begin{align*}
\|\mathcal{I}_m\alpha\|_{L^2(X,\omega,g_m)}^2
=
\int_X |\mathcal{I}_m\alpha(x)|_{\omega,g_m}^2\,dV_\omega(x)
=
\int_X |\alpha(x)|_{\omega,h^m}^2\,dV_\omega(x)
=
\|\alpha\|_{L^2(X,\omega,h^m)}^2.
\end{align*}
Because the identification uses holomorphic tensor factors, it commutes with $\bar\partial$. Thus $\bar\partial f=0$ implies $\bar\partial F_m=0$ for $F_m:=\mathcal{I}_m f$.
[/guided]
[/step]
[step:Show that the curvature term grows at least linearly in $m$]
Let $\Theta_h(L)$ denote the Chern curvature of $(L,h)$. Since $(L,h)$ is positive, the real $(1,1)$-form $i\Theta_h(L)$ is positive. Because $X$ is compact, there exists a constant $\lambda>0$ such that
\begin{align*}
i\Theta_h(L)(\xi,\bar{\xi})\ge \lambda\,\omega(\xi,\bar{\xi})
\end{align*}
for every point $x\in X$ and every tangent vector $\xi\in T_x^{1,0}X$.
Let $\Theta_{g_m}(E_m)$ denote the Chern curvature of $(E_m,g_m)$. By additivity of Chern curvature under tensor products of Hermitian line bundles,
\begin{align*}
i\Theta_{g_m}(E_m)
=
i\Theta(K_X^{-1})+m\,i\Theta_h(L).
\end{align*}
The fixed curvature form $i\Theta(K_X^{-1})$ is smooth on compact $X$, so there exists $B>0$ such that, as Hermitian forms,
\begin{align*}
i\Theta(K_X^{-1})\ge -B\,\omega.
\end{align*}
Hence
\begin{align*}
i\Theta_{g_m}(E_m)\ge (m\lambda-B)\omega.
\end{align*}
For an $E_m$-valued $(n,q)$-form $\alpha$, the commutator $[i\omega,\Lambda_\omega]$ acts as multiplication by $q$. Therefore
\begin{align*}
\bigl([i\Theta_{g_m}(E_m),\Lambda_\omega]\alpha,\alpha\bigr)_{\omega,g_m}
\ge
q(m\lambda-B)|\alpha|_{\omega,g_m}^2.
\end{align*}
Choose $m_0\in\mathbb{N}$ so large that $m\lambda-B\ge \frac{\lambda}{2}m$ for all $m\ge m_0$. Then, for every $m\ge m_0$,
\begin{align*}
\bigl([i\Theta_{g_m}(E_m),\Lambda_\omega]\alpha,\alpha\bigr)_{\omega,g_m}
\ge
\frac{\lambda}{2}m|\alpha|_{\omega,g_m}^2
\end{align*}
because $q\ge 1$.
[guided]
The curvature of $E_m=K_X^{-1}\otimes L^m$ has one part that grows with $m$ and one part that is fixed. The growing part comes from $L^m$.
Let $\Theta_h(L)$ be the Chern curvature of $L$. Positivity of $(L,h)$ means that $i\Theta_h(L)$ is a positive Hermitian $(1,1)$-form. Since $X$ is compact and the smallest eigenvalue of $i\Theta_h(L)$ relative to $\omega$ varies continuously, there is a uniform constant $\lambda>0$ such that
\begin{align*}
i\Theta_h(L)(\xi,\bar{\xi})\ge \lambda\,\omega(\xi,\bar{\xi})
\end{align*}
for all $x\in X$ and all $\xi\in T_x^{1,0}X$.
Curvature is additive under tensor products of Hermitian line bundles. Thus for
\begin{align*}
E_m=K_X^{-1}\otimes L^m
\end{align*}
we have
\begin{align*}
i\Theta_{g_m}(E_m)
=
i\Theta(K_X^{-1})+m\,i\Theta_h(L).
\end{align*}
The curvature $i\Theta(K_X^{-1})$ is independent of $m$. Since it is smooth on compact $X$, its eigenvalues relative to $\omega$ are bounded below. Hence there exists $B>0$ such that
\begin{align*}
i\Theta(K_X^{-1})\ge -B\,\omega.
\end{align*}
Combining this with the lower bound for $i\Theta_h(L)$ gives
\begin{align*}
i\Theta_{g_m}(E_m)
\ge
(m\lambda-B)\omega.
\end{align*}
Now we use the special feature of top holomorphic degree. Let $\Lambda_\omega$ denote the adjoint, with respect to the Kähler metric $\omega$, of exterior multiplication by $\omega$. On $E_m$-valued $(n,q)$-forms, the Lefschetz commutator $[i\omega,\Lambda_\omega]$ acts by the scalar $q$. Therefore, if $\alpha$ is an $E_m$-valued $(n,q)$-form, then
\begin{align*}
\bigl([i\Theta_{g_m}(E_m),\Lambda_\omega]\alpha,\alpha\bigr)_{\omega,g_m}
\ge
q(m\lambda-B)|\alpha|_{\omega,g_m}^2.
\end{align*}
The hypothesis $q\ge 1$ is used exactly here: it gives a strictly positive scalar on top-degree forms. Choose $m_0\in\mathbb{N}$ such that $m\lambda-B\ge \frac{\lambda}{2}m$ whenever $m\ge m_0$. Then
\begin{align*}
\bigl([i\Theta_{g_m}(E_m),\Lambda_\omega]\alpha,\alpha\bigr)_{\omega,g_m}
\ge
\frac{\lambda}{2}m|\alpha|_{\omega,g_m}^2
\end{align*}
for all $m\ge m_0$ and all $E_m$-valued $(n,q)$-forms $\alpha$.
[/guided]
[/step]
[step:Apply the Bochner--Kodaira--Nakano identity to obtain a coercive estimate]
Let
\begin{align*}
A_m:C^\infty\bigl(X;\Lambda^{n,q}T^*X\otimes E_m\bigr)&\to
C^\infty\bigl(X;\Lambda^{n,q+1}T^*X\otimes E_m\bigr),\\
\alpha&\mapsto \bar\partial\alpha
\end{align*}
and let $A_m^*$ denote its $L^2(X,\omega,g_m)$ adjoint on $E_m$-valued forms. The Bochner--Kodaira--Nakano identity for compact Kähler manifolds, applied to the Hermitian holomorphic line bundle $(E_m,g_m)$, gives for every smooth $E_m$-valued $(n,q)$-form $\alpha$:
\begin{align*}
\|A_m\alpha\|_{L^2(X,\omega,g_m)}^2
+
\|A_m^*\alpha\|_{L^2(X,\omega,g_m)}^2
\ge
\int_X
\bigl([i\Theta_{g_m}(E_m),\Lambda_\omega]\alpha(x),\alpha(x)\bigr)_{\omega,g_m}
\,dV_\omega(x).
\end{align*}
Here we are citing a result not yet in the wiki: Bochner--Kodaira--Nakano identity.
Using the curvature lower bound from the previous step, for every $m\ge m_0$ and every smooth $E_m$-valued $(n,q)$-form $\alpha$,
\begin{align*}
\|A_m\alpha\|_{L^2(X,\omega,g_m)}^2
+
\|A_m^*\alpha\|_{L^2(X,\omega,g_m)}^2
\ge
\frac{\lambda}{2}m
\|\alpha\|_{L^2(X,\omega,g_m)}^2.
\end{align*}
[guided]
We now convert curvature positivity into an analytic inequality. Define the operator
\begin{align*}
A_m:C^\infty\bigl(X;\Lambda^{n,q}T^*X\otimes E_m\bigr)&\to
C^\infty\bigl(X;\Lambda^{n,q+1}T^*X\otimes E_m\bigr),\\
\alpha&\mapsto \bar\partial\alpha.
\end{align*}
Let $A_m^*$ be its adjoint with respect to the $L^2$ inner product induced by $\omega$ and $g_m$.
The Bochner--Kodaira--Nakano identity says that, on a compact Kähler manifold and for a Hermitian holomorphic vector bundle, the $\bar\partial$ energy controls the curvature commutator. In the present line-bundle case it gives
\begin{align*}
\|A_m\alpha\|_{L^2(X,\omega,g_m)}^2
+
\|A_m^*\alpha\|_{L^2(X,\omega,g_m)}^2
\ge
\int_X
\bigl([i\Theta_{g_m}(E_m),\Lambda_\omega]\alpha(x),\alpha(x)\bigr)_{\omega,g_m}
\,dV_\omega(x).
\end{align*}
Here we are citing a result not yet in the wiki: Bochner--Kodaira--Nakano identity.
The previous step proved the pointwise estimate
\begin{align*}
\bigl([i\Theta_{g_m}(E_m),\Lambda_\omega]\alpha(x),\alpha(x)\bigr)_{\omega,g_m}
\ge
\frac{\lambda}{2}m|\alpha(x)|_{\omega,g_m}^2
\end{align*}
for every $x\in X$. Integrating this pointwise inequality with respect to the Kähler volume measure $dV_\omega(x)$ gives
\begin{align*}
\int_X
\bigl([i\Theta_{g_m}(E_m),\Lambda_\omega]\alpha(x),\alpha(x)\bigr)_{\omega,g_m}
\,dV_\omega(x)
\ge
\frac{\lambda}{2}m
\int_X|\alpha(x)|_{\omega,g_m}^2\,dV_\omega(x).
\end{align*}
Therefore
\begin{align*}
\|A_m\alpha\|_{L^2(X,\omega,g_m)}^2
+
\|A_m^*\alpha\|_{L^2(X,\omega,g_m)}^2
\ge
\frac{\lambda}{2}m
\|\alpha\|_{L^2(X,\omega,g_m)}^2.
\end{align*}
This is the coercive estimate: the $\bar\partial$ complex has a spectral gap in degree $q$ of size at least
\begin{align*}
\frac{\lambda}{2}m.
\end{align*}
[/guided]
[/step]
[step:Use the coercive estimate to solve $\bar\partial U_m=F_m$ with the sharp $m^{-1}$ bound]
For each integer $r\ge 0$, let $H_{m,r}$ be the [Hilbert space](/page/Hilbert%20Space) completion of smooth $E_m$-valued $(n,r)$-forms in the norm $\|\cdot\|_{L^2(X,\omega,g_m)}$. Define the closed densely defined operator
\begin{align*}
D_{m,r}:\operatorname{Dom}(D_{m,r})\subset H_{m,r}&\to H_{m,r+1} \\
\alpha&\mapsto \bar\partial\alpha
\end{align*}
as the maximal closed extension of $\bar\partial$ from smooth forms, and let $D_{m,r}^*$ denote its Hilbert-space adjoint. The estimate from the previous step extends by density to every
\begin{align*}
\alpha\in \operatorname{Dom}(D_{m,q})\cap \operatorname{Dom}(D_{m,q-1}^*)
\end{align*}
and gives
\begin{align*}
\|D_{m,q}\alpha\|_{L^2(X,\omega,g_m)}^2
+
\|D_{m,q-1}^*\alpha\|_{L^2(X,\omega,g_m)}^2
\ge
\frac{\lambda}{2}m\|\alpha\|_{L^2(X,\omega,g_m)}^2.
\end{align*}
Set
\begin{align*}
c_m:=\frac{\lambda}{2}m.
\end{align*}
We now use the standard closed-range estimate for Hilbert complexes (citing a result not yet in the wiki: closed-range minimal-solution theorem for Hilbert complexes). In the following form, it says that for a closed Hilbert complex
\begin{align*}
H_{m,q-1}\xrightarrow{D_{m,q-1}}H_{m,q}\xrightarrow{D_{m,q}}H_{m,q+1},
\end{align*}
if the estimate
\begin{align*}
c_m\|\alpha\|_{L^2(X,\omega,g_m)}^2
\le
\|D_{m,q}\alpha\|_{L^2(X,\omega,g_m)}^2
+
\|D_{m,q-1}^*\alpha\|_{L^2(X,\omega,g_m)}^2
\end{align*}
holds for all $\alpha\in \operatorname{Dom}(D_{m,q})\cap\operatorname{Dom}(D_{m,q-1}^*)$, then $\operatorname{Ran}D_{m,q-1}$ is closed, $\ker D_{m,q}=\operatorname{Ran}D_{m,q-1}$, and every $F\in\ker D_{m,q}$ has a unique minimal solution
\begin{align*}
U\in \operatorname{Dom}(D_{m,q-1})\cap(\ker D_{m,q-1})^\perp
\end{align*}
such that
\begin{align*}
D_{m,q-1}U=F
\end{align*}
and
\begin{align*}
\|U\|_{L^2(X,\omega,g_m)}^2\le c_m^{-1}\|F\|_{L^2(X,\omega,g_m)}^2.
\end{align*}
In the Dolbeault complex, the same theorem gives the weak normal condition
\begin{align*}
D_{m,q-2}^*U=0
\end{align*}
for the minimal solution, with this term omitted when $q=1$. This quoted Hilbert-complex theorem packages the domain statements, closed-range conclusion, normal equation, and minimal-norm estimate; no additional differentiation of a weak Laplacian equation is being used here.
Since $F_m$ is smooth and $\bar\partial F_m=0$, we have $F_m\in\ker D_{m,q}$. Applying the closed-range theorem gives an $E_m$-valued $(n,q-1)$-form
\begin{align*}
U_m\in\operatorname{Dom}(D_{m,q-1})
\end{align*}
such that
\begin{align*}
\bar\partial U_m=D_{m,q-1}U_m=F_m
\end{align*}
and
\begin{align*}
\|U_m\|_{L^2(X,\omega,g_m)}^2
\le
\frac{2}{\lambda m}\|F_m\|_{L^2(X,\omega,g_m)}^2.
\end{align*}
[guided]
The coercive estimate must be applied to a precise Hilbert complex, not to an ambiguous symbol indexed by form degree. For each integer $r\ge 0$, define $H_{m,r}$ to be the $L^2$ Hilbert space of $E_m$-valued $(n,r)$-forms, obtained by completing smooth forms in the norm induced by $\omega$ and $g_m$. Define
\begin{align*}
D_{m,r}:\operatorname{Dom}(D_{m,r})\subset H_{m,r}&\to H_{m,r+1} \\
\alpha&\mapsto \bar\partial\alpha
\end{align*}
to be the maximal closed extension of $\bar\partial$, and let $D_{m,r}^*$ be its Hilbert-space adjoint. These operators are densely defined because smooth forms are dense in each $H_{m,r}$.
The previous step proved the estimate first for smooth $E_m$-valued $(n,q)$-forms. Since $D_{m,q}$ and $D_{m,q-1}^*$ are closed and smooth forms form a core for the Dolbeault Laplacian on the compact Kähler manifold $X$, the same estimate extends to every
\begin{align*}
\alpha\in \operatorname{Dom}(D_{m,q})\cap \operatorname{Dom}(D_{m,q-1}^*)
\end{align*}
as
\begin{align*}
\|D_{m,q}\alpha\|_{L^2(X,\omega,g_m)}^2
+
\|D_{m,q-1}^*\alpha\|_{L^2(X,\omega,g_m)}^2
\ge
\frac{\lambda}{2}m\|\alpha\|_{L^2(X,\omega,g_m)}^2.
\end{align*}
Set
\begin{align*}
c_m:=\frac{\lambda}{2}m.
\end{align*}
This estimate is the closed-range input for solving $\bar\partial$. More explicitly, on the Hilbert complex
\begin{align*}
H_{m,q-1}\xrightarrow{D_{m,q-1}}H_{m,q}\xrightarrow{D_{m,q}}H_{m,q+1},
\end{align*}
we use the standard closed-range minimal-solution theorem for Hilbert complexes. It says that the displayed estimate implies
\begin{align*}
\ker D_{m,q}=\operatorname{Ran}D_{m,q-1}
\end{align*}
and that every $F\in\ker D_{m,q}$ has a unique solution
\begin{align*}
U\in \operatorname{Dom}(D_{m,q-1})\cap(\ker D_{m,q-1})^\perp
\end{align*}
with
\begin{align*}
D_{m,q-1}U=F,
\qquad
\|U\|_{L^2(X,\omega,g_m)}^2\le c_m^{-1}\|F\|_{L^2(X,\omega,g_m)}^2.
\end{align*}
For the minimal solution in the Dolbeault complex, the same theorem also gives
\begin{align*}
D_{m,q-2}^*U=0,
\end{align*}
with the term omitted when $q=1$.
This is the advertised minimal-solution estimate.
The form $F_m$ is smooth and satisfies $\bar\partial F_m=0$, so $F_m\in\operatorname{Dom}(D_{m,q})$ and $D_{m,q}F_m=0$. Therefore the Hilbert-space theorem gives an $E_m$-valued $(n,q-1)$-form
\begin{align*}
U_m\in\operatorname{Dom}(D_{m,q-1})
\end{align*}
such that
\begin{align*}
\bar\partial U_m=D_{m,q-1}U_m=F_m
\end{align*}
and
\begin{align*}
\|U_m\|_{L^2(X,\omega,g_m)}^2
\le
\frac{1}{c_m}\|F_m\|_{L^2(X,\omega,g_m)}^2
=
\frac{2}{\lambda m}\|F_m\|_{L^2(X,\omega,g_m)}^2.
\end{align*}
The positivity estimate eliminates harmonic obstructions in degree $q$, and the operator $D_{m,q-1}$ is the correctly typed previous-degree $\bar\partial$ operator with the same coefficient bundle $E_m$.
[/guided]
[/step]
[step:Return to $(0,q-1)$-forms and apply elliptic regularity]
Define
\begin{align*}
u:=\mathcal{I}_m^{-1}U_m.
\end{align*}
Then $u$ is an $L^m$-valued $(0,q-1)$-form and, because $\mathcal{I}_m$ commutes with $\bar\partial$,
\begin{align*}
\bar\partial u
=
\mathcal{I}_m^{-1}(\bar\partial U_m)
=
\mathcal{I}_m^{-1}F_m
=
f.
\end{align*}
We chose $U_m$ as the minimal $L^2$ solution supplied by the closed-range theorem. Thus it also satisfies the weak normal condition
\begin{align*}
D_{m,q-2}^*U_m=0
\end{align*}
with this term omitted when $q=1$. Together with $D_{m,q-1}U_m=F_m$, this gives the elliptic system for a minimal $\bar\partial$ solution. Define the degree-$(q-1)$ Dolbeault Laplacian by
\begin{align*}
\Box_{m,q-1}:=
D_{m,q-1}^*D_{m,q-1}
+
D_{m,q-2}D_{m,q-2}^*,
\end{align*}
with the second summand omitted when $q=1$. Equivalently, in the weak sense,
\begin{align*}
\Box_{m,q-1}U_m=D_{m,q-1}^*F_m.
\end{align*}
The right-hand side is smooth because $F_m$ is smooth and $D_{m,q-1}^*$ is a first-order differential operator with smooth coefficients. Elliptic regularity for the Dolbeault Laplacian on compact Hermitian manifolds (citing a result not yet in the wiki: elliptic regularity for the Dolbeault Laplacian) therefore implies that $U_m$ is smooth. Hence $u$ is smooth.
Finally, since $\mathcal{I}_m$ is an isometry,
\begin{align*}
\|u\|_{L^2(X,\omega,h^m)}^2
=
\|U_m\|_{L^2(X,\omega,g_m)}^2
\le
\frac{2}{\lambda m}
\|F_m\|_{L^2(X,\omega,g_m)}^2
=
\frac{2}{\lambda m}
\|f\|_{L^2(X,\omega,h^m)}^2.
\end{align*}
Taking
\begin{align*}
C:=\frac{2}{\lambda}
\end{align*}
proves the desired estimate. The integer $m_0$ depends only on the lower positivity constant $\lambda$ and the fixed curvature bound $B$ for $K_X^{-1}$, not on $m$, $q$, or $f$; the constant $C=2/\lambda$ is independent of $m$ and $f$.
[guided]
We now convert the solution back to the form type in the theorem. Define
\begin{align*}
u:=\mathcal{I}_m^{-1}U_m.
\end{align*}
Since $U_m$ is an $E_m$-valued $(n,q-1)$-form and $\mathcal{I}_m$ identifies $L^m$-valued $(0,q-1)$-forms with $E_m$-valued $(n,q-1)$-forms, the form $u$ is an $L^m$-valued $(0,q-1)$-form.
Because $\mathcal{I}_m$ commutes with $\bar\partial$, the equation for $U_m$ becomes the desired equation for $u$:
\begin{align*}
\bar\partial u
=
\mathcal{I}_m^{-1}(\bar\partial U_m)
=
\mathcal{I}_m^{-1}F_m
=
f.
\end{align*}
It remains to justify smoothness. The Hilbert-space theorem gives the minimal $L^2$ solution $U_m$, and minimality supplies the weak normal equation
\begin{align*}
D_{m,q-2}^*U_m=0
\end{align*}
with the term omitted for $q=1$. Since $D_{m,q-1}U_m=F_m$, the pair of equations
\begin{align*}
D_{m,q-1}U_m &= F_m, &
D_{m,q-2}^*U_m &= 0
\end{align*}
is an elliptic first-order system for $U_m$. Let
\begin{align*}
\Box_{m,q-1}:=
D_{m,q-1}^*D_{m,q-1}
+
D_{m,q-2}D_{m,q-2}^*,
\end{align*}
again omitting the second summand when $q=1$. Equivalently,
\begin{align*}
\Box_{m,q-1}U_m=D_{m,q-1}^*F_m
\end{align*}
in the weak sense. The right-hand side is smooth because $F_m$ is smooth. Elliptic regularity for the Dolbeault Laplacian on compact Hermitian manifolds gives that $U_m$ is smooth. Hence $u=\mathcal{I}_m^{-1}U_m$ is smooth.
Finally, the isometry property of $\mathcal{I}_m$ transfers the estimate without changing the constant:
\begin{align*}
\|u\|_{L^2(X,\omega,h^m)}^2
=
\|U_m\|_{L^2(X,\omega,g_m)}^2
\le
\frac{2}{\lambda m}
\|F_m\|_{L^2(X,\omega,g_m)}^2
=
\frac{2}{\lambda m}
\|f\|_{L^2(X,\omega,h^m)}^2.
\end{align*}
Thus the theorem holds with
\begin{align*}
C:=\frac{2}{\lambda}.
\end{align*}
The integer $m_0$ depends only on the positivity constant $\lambda$ and the fixed curvature bound $B$ for $K_X^{-1}$, not on $m$, $q$, or $f$; the constant $C$ is independent of $m$ and $f$.
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