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I'm trying to prove Chow's theorem (closed analytic subvarieties of projective space are algebraic) in a special case by the following differential geometric method (imitating the argument for the Kodaira embedding theorem in Griffiths-Harris).

I want to show the following: let $X$ be a compact complex manifold, $L$ a positive line bundle on it, $V\subset X$ a compact complex submanifold and let $x\in X$ be a point not in $V$. Then for $n\gg0$, there exists a section $\sigma\in H^0(X,L^{\otimes n})$ such that $\sigma|_V\equiv 0$ and $\sigma(x)\ne 0$.

The strategy I have in mind is as follows. If $V$ is of codimension $1$, then the ideal sheaf of $V$ defines a line bundle $\mathcal O_X(-V)$, and thus, $L^{\otimes n}\otimes\mathcal O_{X}(-V)$ is positive for $n\gg 0$ (and thus, is globally generated for $n\gg 0$), which proves the result. If $V$ is of codimension $\ge 2$, then let $\pi:Y\to X$ be the blow-up of $X$ along $V$, and let $E$ be the exceptional divisor. We know that $E \cong \mathbb P(N_{X}V)$, and that $\mathcal O_Y(E)|_E = N_{E}Y$ is the relative $\mathcal O(-1)$ bundle on $E\to V$. Now, $\mathcal O_Y(-E)$ is positive on each fibre of $E\to V$ (and these are the directions contracted by the blowdown map $\pi$). So, I'm led to guess that $\tilde L = \pi^*L^{\otimes n}\otimes\mathcal O_Y(-E)$ is positive for $n\gg 0$ (I have not yet done the computation to check this). If this is the case, then we are again done.

So my question is: is $\tilde L$ positive for $n\gg 0$?

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    $\begingroup$ Why not just consider $L^{\otimes n}\otimes\mathcal{I}_V$? $\endgroup$
    – Chen Jiang
    Commented Apr 28, 2018 at 14:58
  • $\begingroup$ The reason I don't want to do that is because I want to work entirely with line bundles if possible (rather than more general coherent sheaves). In that case the machinery of Hodge theory and harmonic forms becomes applicable. $\endgroup$ Commented Apr 28, 2018 at 17:33
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    $\begingroup$ In general $\widetilde{L}$ is nef (Demailly, Peternernell, Schneider - Compact Complex Manifolds with Numerically Effective Tangent Bundles, J. Alg. Geom. 3 (1994) 295-345; definition 1.2 and proposition 1.8.(i)); you can consider the Hermitian metric $h$ on $\widetilde{L}$ and to check whether its Chern curvature is definite positive everywhere. $\endgroup$ Commented Apr 30, 2018 at 11:23
  • $\begingroup$ You can find a proof on page 357 of Demailly's book Complex Analytic and Differential Geometry $\endgroup$ Commented Mar 20, 2023 at 16:01

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