[step:Bound the correction by the weighted $L^2$ estimate, where $\alpha>1$ produces the factor $1/(\alpha-1)$]If $q=0$ then $m=1$ and $S=\{0\}$; here $\sigma=f/g_1$ is already holomorphic on $\Omega'$, so $\bar\partial\sigma=0$ and we take $u\equiv 0$, for which the estimate below holds with right-hand side $0$. Assume henceforth $q\ge 1$, so that $\beta=\alpha q>q$.
Equip $\Omega\subset\mathbb C^n$ with the standard Kähler form
\begin{align*}
\omega:=i\sum_{k=1}^n dz_k\wedge d\bar z_k,
\end{align*}
and let $\Lambda$ denote the pointwise adjoint of the Lefschetz operator $\tau\mapsto\omega\wedge\tau$ with respect to the induced Hermitian inner product on forms (the Lefschetz contraction). Define the plurisubharmonic weight
\begin{align*}
\Phi:\Omega\to[-\infty,\infty),\qquad \Phi:=\varphi+\beta\log|g|^2,
\end{align*}
so that $e^{-\Phi}=e^{-\varphi}|g|^{-2\beta}$, and the Hermitian curvature endomorphism acting on $S$-valued $(0,1)$-forms
\begin{align*}
B:=[\,\Theta,\ \Lambda\,],\qquad \Theta:=i\partial\bar\partial\Phi\otimes\mathrm{Id}_S+\Theta_S,
\end{align*}
where $\Theta_S$ is the Chern curvature of the subbundle $S\hookrightarrow\Omega\times\mathbb C^m$. Since the ambient bundle $\Omega\times\mathbb C^m$ carries the flat product connection, its curvature vanishes, and the Gauss–Codazzi relation for a holomorphic subbundle, $\Theta_S=\Theta_{\Omega\times\mathbb C^m}\big|_S-\beta_S^*\wedge\beta_S$, reduces to $\Theta_S=-\beta_S^*\wedge\beta_S$, where $\beta_S$ is the second fundamental form of $S$ in $\Omega\times\mathbb C^m$; the second fundamental form thus lowers the curvature, and this sign is what the estimate below relies on. The derivation of this relation and of the resulting curvature bound is precisely the content of Skoda's pointwise inequality stated next.
We invoke two results. First, the **weighted $L^2$ existence theorem for $\bar\partial$ with curvature lower bound** (Bochner–Kodaira–Nakano/Hörmander), which we state in full as the result being applied: if $\Omega$ is pseudoconvex, $\Phi$ is plurisubharmonic, and $B$ is positive definite on $S$-valued $(0,1)$-forms, then for every $\bar\partial$-closed $S$-valued $(0,1)$-form $w$ with $\int_\Omega\langle B^{-1}w,w\rangle\,e^{-\Phi}\,d\mathcal L^{2n}<\infty$ there is a section $u$ of $S$ with $\bar\partial u=w$ and
\begin{align*}
\int_\Omega |u|^2 e^{-\Phi}\,d\mathcal L^{2n}\le\int_\Omega\langle B^{-1}w,w\rangle\, e^{-\Phi}\,d\mathcal L^{2n}.
\end{align*}
We verify its hypotheses: $\Omega$ is pseudoconvex (given); $\Phi=\varphi+\beta\log|g|^2$ is plurisubharmonic, since $\varphi$ is plurisubharmonic (given) and $\log|g|^2$ is the composition of the plurisubharmonic function $\zeta\mapsto\log|\zeta|^2$ on $\mathbb C^m$ with the holomorphic map $g$, hence plurisubharmonic ([Log-Modulus of a Holomorphic Function is PSH](/theorems/3405), [Stability Properties of PSH Functions](/theorems/3404)); the datum $w:=\bar\partial\sigma$ is $\bar\partial$-closed and $S$-valued by Step 2; and positivity of $B$ is supplied by the second result. The crude unweighted bound $\|u\|_{L^2}\le C_\Omega\|w\|_{L^2}$ furnished by [$\bar\partial$-Solvability on Pseudoconvex Domains](/theorems/3493) does not suffice, as its constant $C_\Omega$ is not explicit and cannot produce the sharp factor $\alpha/(\alpha-1)$; the curvature form of the estimate is essential.
Second, **Skoda's pointwise curvature inequality for a surjective bundle morphism**, which we likewise state in full as the result being applied: when $\beta>q$, the endomorphism $B$ is positive definite on $S$-valued $(0,1)$-forms and
\begin{align*}
\langle B^{-1}\,\bar\partial\sigma,\bar\partial\sigma\rangle\le\frac{q}{\beta-q}\,\frac{|f|^2}{|g|^2}\qquad\text{pointwise on }\Omega'.
\end{align*}
The number $q=\min\{n,m-1\}$ is the maximal rank of $i\partial\bar\partial\log|g|^2$ (it cannot exceed the dimension $n$ of $\Omega$, nor the rank $m-1$ of the relation bundle), and $\beta-q=q(\alpha-1)>0$ is exactly the gap created by $\alpha>1$.
Combining the two ingredients with $w=\bar\partial\sigma$ and $e^{-\Phi}=e^{-\varphi}|g|^{-2\beta}$, for any smooth plurisubharmonic weight the existence theorem furnishes a solution $u$ of $\bar\partial u=\bar\partial\sigma$ with
\begin{align*}
\int_{\Omega}\frac{|u|^2e^{-\varphi}}{|g|^{2\beta}}\,d\mathcal L^{2n}
\le\frac{q}{\beta-q}\int_{\Omega}\frac{|f|^2e^{-\varphi}}{|g|^{2(\beta+1)}}\,d\mathcal L^{2n}
=\frac{q}{\beta-q}\,I,
\end{align*}
and with $\beta=\alpha q$, $q\ge 1$ we get $\dfrac{q}{\beta-q}=\dfrac{q}{\alpha q-q}=\dfrac{1}{\alpha-1}$. This is where the hypothesis $\alpha>1$ is consumed. Because $\Phi$ is singular along $Z$ (where $\log|g|^2=-\infty$) and $\varphi$ need not be smooth, the existence theorem is applied not to $\Phi$ directly but to the smooth plurisubharmonic approximations of Step 4, which recover this bound in the limit.[/step]