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Dolbeault h zero one of the riemann sphere vanishes
Example
Assume the Axiom of Choice (The Axiom of Choice), inherited through the harmonic-star duality and finiteness items used below. Let be the Riemann sphere with its holomorphic charts (The standard holomorphic charts on the Riemann sphere, with holomorphy and poles at infinity, Stereographic projection identifies the Riemann sphere with the unit two-sphere, The Riemann sphere is the published one-point compactification of the complex plane), let be the trivial holomorphic line bundle with the constant Hermitian metric, and let be any compatible Riemannian metric. Then
so the Dolbeault group of the trivial bundle on the sphere vanishes in every degree ; equivalently the space of holomorphic differentials is zero, so there is no nonzero holomorphic -form on the sphere. In particular every -closed -form on the sphere is -exact.
Facts & Assumptions
Given: The compact Riemann sphere, the trivial holomorphic line bundle with constant positive weight, any compatible metric, and full AC.
The sphere has charts on and about infinity. A holomorphic differential has holomorphic chart coefficients with the differential transition law (The standard holomorphic charts on the Riemann sphere, with holomorphy and poles at infinity, Meromorphic differentials, orders and residues, Holomorphic line bundles and meromorphic sections on a Riemann surface).
Every bounded entire function is constant (Liouville's theorem: every bounded entire function is constant).
Harmonic-star duality is a conjugate-linear isomorphism ; Dolbeault cohomology is identified with harmonic representatives, and degree zero is the holomorphic-section space (Harmonic star duality for line bundle valued dolbeault cohomology, Dolbeault cohomology of a compact riemann surface is finite dimensional, Hodge decomposition for Dolbeault forms on a compact Riemann surface).
Verification
Given: The data in the Example and Facts.
A holomorphic function on the sphere is entire in the chart and bounded near infinity, since its -chart expression is continuous at . It is also bounded on each closed disk, so is bounded on all of . By [F2] it is constant, and all constants are global holomorphic sections of the trivial bundle. Thus [F3] gives .
Write a global holomorphic differential as on , with entire. In the other chart it is , where for by [F1]. Holomorphy at bounds on a small closed disk. Consequently for sufficiently large . The entire function is bounded on a closed disk and on its exterior, so [F2] makes it constant; the displayed decay forces that constant to vanish. Therefore .
The trivial dual bundle identifies holomorphically with . By [F3] and step 1.2, the degree-one harmonic space and hence are zero. Every smooth -form on a curve is closed because there are no -forms; its zero cohomology class says exactly that it is of a global smooth function. This proves the stated exactness for any supplied compatible metric. Full AC is inherited through duality and the harmonic representative interfaces.
Source notes
Demailly’s Dolbeault results in §7 apply to a holomorphic Hermitian bundle; the flat-connection de Rham decomposition in §3.3 is contextual and is not used for a varying Hermitian weight. The verification above uses proved local operator interfaces and gives the concrete calculation itself.
Depends on
- Dolbeault cohomology of a compact riemann surface is finite dimensional
- The Axiom of Choice
- The chordal metric on the Riemann sphere
- A complex domain is a nonempty connected open subset of $\mathbb C$
- Hermitian metric and $L^2$ pairing on a compact Riemann surface
- Holomorphic line bundles and meromorphic sections on a Riemann surface
- Isolated singularities: removable, poles, and essential singularities
- Meromorphic differentials, orders and residues
- The standard holomorphic charts on the Riemann sphere, with holomorphy and poles at infinity
- The Riemann sphere is the published one-point compactification of the complex plane
- Harmonic star duality for line bundle valued dolbeault cohomology
- Hodge decomposition for Dolbeault forms on a compact Riemann surface
- Liouville's theorem: every bounded entire function is constant
- Meromorphic functions on the Riemann sphere are exactly the rational functions
- Stereographic projection identifies the Riemann sphere with the unit two-sphere
Used by
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Sources
- Jean-Pierre Demailly, Complex Analytic and Differential Geometry (author manuscript, Universite Grenoble Alpes) (standard reference, not scraped)