Holomorphic vector bundle

A holomorphic vector bundle is a complex vector bundle whose local trivializations have holomorphic transition functions.

A unitary representation of the fundamental group of a connected complex manifold gives a bundle $(\tilde{X}\times V)/{\pi }_{1}X$ with locally constant unitary transition matrices. Its induced flat unitary connection has $(0,1)$ part equal to its Dolbeault operator, so it is the Chern connection for the induced Hermitian metric. On a compact Riemann surface, zero curvature gives degree zero; the subbundle curvature inequality makes the bundle semistable.

In a local parallel orthonormal frame a holomorphic section has holomorphic coefficients $f_j$. Its squared norm satisfies ${\partial }_z{\partial }_{\bar{z}}{\sum }_j|f_j{|}^2={\sum }_j|f_j^{\prime }{|}^2\ge 0$. On a compact connected complex curve, integration of the Laplacian makes the right side zero. Thus every coefficient is locally constant and the section is parallel. A global parallel section is determined by a vector fixed by all monodromy operators, and every such vector gives a global section.

On a compact Riemann surface, the degree of a holomorphic vector bundle is the degree of a line bundle of its determinant line bundle. A Hermitian metric on a holomorphic vector bundle and its Chern connection express this degree by the trace-curvature integral. The value is independent of the metric because the normalized trace curvature represents the First Chern class.

The slope of a nonzero holomorphic vector bundle on a compact Riemann surface is its degree of a holomorphic vector bundle divided by its rank of a vector bundle. It measures degree per fibre dimension. Comparing subbundle slopes defines semistable holomorphic vector bundles.

Fix a Hermitian metric on a holomorphic vector bundle $E$ over a compact Riemann surface and a positive area form $\omega$. Write $i{F}_E=K\omega$ for its Chern curvature. If every eigenvalue of the Hermitian endomorphism $K$ is at most $C$, the curvature formula for a holomorphic subbundle gives $\mathrm{deg}F\le \mathrm{rank}(F)C{\int }_X\omega /(2\pi )$. The second-fundamental-form term is nonpositive. Compactness supplies a finite $C$ independent of the subbundle. If $F_E=0$, every subbundle has nonpositive degree.

A positive-rank holomorphic vector bundle on a compact Riemann surface is semistable when every nonzero proper holomorphic subbundle has slope at most its own. Equivalently one tests coherent subsheaves: saturation on a smooth curve preserves rank, increases degree, and gives a subbundle with torsion-free quotient. Strict inequality defines stability, which is stronger.

For a holomorphically trivial vector bundle, choose a global holomorphic bundle frame. Let the columns of $H_{\alpha }$ be the coordinates of that frame in the prescribed frame over $U_{\alpha }$. If fiber coordinates transform as $s_{\beta }=F_{\alpha \beta }{s}_{\alpha }$, then $H_{\beta }=F_{\alpha \beta }{H}_{\alpha }$. Each matrix is holomorphic and invertible, giving the displayed splitting. Conversely such matrices give agreeing global holomorphic sections forming a frame. Topological triviality alone does not supply this conclusion.

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A holomorphic subbundle of a holomorphic vector bundle is a constant-rank vector subbundle which is locally spanned by a holomorphic linearly independent family of sections and whose inclusion is a holomorphic bundle map. It has an induced holomorphic vector bundle structure. A varying-rank coherent subsheaf is not generally a subbundle.

Dualize the fibres of a holomorphic vector bundle. If its transitions are $g_{ij}$, the dual transitions are $g_{ij}^{-T}$, which are holomorphic and preserve the evaluation pairing. These define the natural holomorphic structure on the smooth dual bundle.

For a holomorphic vector bundle $E$, $\mathcal{O}(E)(U)$ is its space of holomorphic sections on $U$, with their restrictions. It is the kernel of the bundle Dolbeault operator on smooth sections.

A holomorphic line bundle is a complex line bundle with holomorphic local trivializations and holomorphic nonvanishing transition functions.

A global holomorphic section of $\mathcal{O}(1)$ on ${\mathbb{CP}}^1$ is a linear homogeneous polynomial in the two projective coordinates. On one patch its coefficient is an entire function $p(\lambda )$; regularity on the other patch says $p(\lambda )/\lambda$ is regular at infinity. Hence $p$ has at most linear growth. Cauchy estimates make its second and higher derivatives zero, so $p=a+b\lambda$. The same argument entrywise applies to matrix-valued sections and makes the logarithmic derivative in the Penrose-Ward correspondence linear in the primed two-component spinor.

On a compact connected complex manifold, pair nonzero holomorphic sections of a holomorphic line bundle and its holomorphic dual vector bundle. Their product is a global holomorphic function, hence constant by the maximum modulus principle. The identity theorem makes their nonvanishing loci dense, so this constant is nonzero. Both sections are consequently nowhere zero and trivialize their bundles. Conversely, the constant section of a trivial bundle and its dual provide the two sections.

A nowhere-zero smooth section $s$ of a holomorphic line bundle writes its Dolbeault operator as ${\bar{\partial }}_{L}s=a\otimes s$. Its integrability gives $\bar{\partial }a=0$. If $H^{0,1}(X)=0$, then $a=\bar{\partial }f$ globally. The section $e^{-f}s$ is nowhere zero and holomorphic. This argument requires no compactness and explicitly constructs the holomorphic trivialization.

In a holomorphic local frame, a meromorphic section has a meromorphic scalar coefficient; the coefficients transform by the bundle's transition functions of a vector bundle. A nonzero meromorphic section has a divisor on a complex manifold recording its zero and pole orders.

For a holomorphic line bundle on a complex curve, a meromorphic section modulo sections holomorphic through a point is its principal part. In a local coordinate and holomorphic local frame it is represented by finitely many negative powers. A vanishing $H^1(X,\mathcal{O}(E))$ permits prescribed principal parts, by a cutoff followed by solving a global Dolbeault operator equation.

For the structure sheaf of a complex manifold, local holomorphic logarithms give the short exact sequence of sheavesThe connecting map $H^1(M,{\mathcal{O}}_M^{\ast })\to H^2(M,\mathbb{Z})$ sends a holomorphic line bundle to its First Chern class. In particular, it is onto when $H^2(M,{\mathcal{O}}_M)=0$.

A Dolbeault partial connection obeys ${\bar{\partial }}_E(fs)=\bar{\partial }f\otimes s+f{\bar{\partial }}_{E}s$. Its square is a tensorial $(0,2)$-form with values in $\mathrm{End}E$; it vanishes for the canonical partial connection of a holomorphic bundle.

A Hermitian metric on a holomorphic vector bundle $E\to X$ is a smoothly varying positive-definite Hermitian form $h_x$ on every complex fiber $E_x$.

Such a metric makes the underlying smooth bundle a Hermitian vector bundle.

A holomorphic line bundle is semipositive if it admits a smooth Hermitian metric whose normalized Chern curvature form is semipositive. Tensoring a positive holomorphic line bundle with a nonnegative integer power of a semipositive one remains positive, because their curvature forms add. This smooth-metric property is stronger than simply having a nef numerical class.

With a Hermitian form linear in its first slot, $H(s\otimes \bar{t})=h(s,t)$ is a complex-linear dual tensor. The original connection, its conjugate connection, the tensor product connection and the dual connection induce $D_0$. Its derivative is $(D_{0,V}H)(s\otimes \bar{t})=Vh(s,t)-h(D_{V}s,t)-h(s,D_{V}t)$. Consequently $D_{0}H=0$ is exactly metric compatibility.

For a smooth subbundle $S$ of a Hermitian vector bundle $E$, orthogonal projection identifies $E/S$ smoothly with $S^{\perp }$. The quotient metric is $h_Q([v],[w])=h_E((I-{\pi }_S)v,(I-{\pi }_S)w)$. It is independent of representatives and positive definite. For a holomorphic subbundle the resulting smooth quotient metric is defined on the holomorphic quotient, but the orthogonal identification itself need not be holomorphic.

A holomorphic line bundle is positive when it admits a Hermitian metric whose Chern connection has positive curvature form $i{F}_{\nabla }$, equivalently when $i{F}_{\nabla }$ is a positive real (1, 1)-form.

The positive-curvature definition uses a positive real (1, 1)-form.

On a compact Kähler manifold $X$, if $P$ is a positive holomorphic line bundle, then $H^i(X,K_X\otimes P)=0$ for $i>0$. The canonical-bundle twist is part of the hypothesis and conclusion; positivity alone does not remove it on a general manifold. For a complex torus, the canonical bundle is trivial, so higher cohomology of a positive line bundle vanishes without an additional twist.

Let $L$ be positive and $V$ a closed smooth hypersurface in a complex torus. The semipositive canonical bundle of a submanifold of a complex torus makes $P_a=(L\otimes \mathcal{O}(aV)){|}_V$ positive. Since $K_V=\mathcal{O}(V){|}_V$, the restriction of the $a$-th ambient twist is $K_V\otimes P_{a-1}$ when $a\ge 1$. Apply the Kodaira vanishing theorem on $V$, and use $0\to L((a-1)V)\to L(aV)\to L(aV){|}_V\to 0$ and its long exact sequence to induct from the positive bundle $L$ on the torus. No unproved ambient positivity of $\mathcal{O}(V)$ is needed.

For a positive Hermitian holomorphic line bundle on compact $X$, using its curvature as the Kähler form, a linear lower bound in $k$ for the full Dolbeault Laplacian on positive antiholomorphic degrees rules out harmonic forms for sufficiently large $k$. The Dolbeault theorem and Dolbeault Hodge decomposition then imply $H^{q>0}(X,L^k)=0$.

The Kodaira embedding theorem says that a compact complex manifold carrying a positive holomorphic line bundle is projective: sufficiently large tensor powers of the bundle have global holomorphic sections defining a holomorphic embedding into Complex projective space. Conversely, the restriction of $\mathcal{O}(1)$ gives a positive line bundle on every projective complex manifold.

A holomorphic local trivialization of a rank-$r$ holomorphic vector bundle $E\to X$ is a biholomorphic bundle map $E{|}_U\to U\times {\mathbb{C}}^r$ that is complex linear on each fiber. Equivalently, it is a local frame of holomorphic sections.

A section of a holomorphic vector bundle is holomorphic when its coordinate functions in every holomorphic local trivialization are holomorphic.

It is a section of a fiber bundle satisfying holomorphicity.

A section of $\mathcal{O}(1)\oplus \mathcal{O}(1)$ on the complex projective line is a pair of homogeneous linear forms ${\ell }_1=a{Z}_0+b{Z}_1$, ${\ell }_2=c{Z}_0+d{Z}_1$. It vanishes at exactly one projective point precisely when its coefficient matrix has rank one, equivalently $ad-bc=0$ and the matrix is nonzero. Its kernel is then one line in ${\mathbb{C}}^2$. Equivalently, $s=v\ell$ for a constant nonzero vector $v$ and one nonzero linear form $\ell$. Rank two gives no zero, while rank zero gives the identically zero section.