Math 2.0

Several complex variables and analytic spaces Definition

Coherent analytic sheaf

Builds on Complex analytic spaceSheaf of modules

Definition

Let (X,π’ͺX)(X, \mathcal{O}_X) be a complex analytic space and let β„±\mathcal{F} be a sheaf of π’ͺX\mathcal{O}_X-modules. The sheaf β„±\mathcal{F} is of finite type if each point of XX has an open neighbourhood UU with a surjective morphism π’ͺUpβ†’β„±βˆ£U\mathcal{O}_U^p \to \mathcal{F}|_U for some pβ‰₯0p \ge 0. The sheaf β„±\mathcal{F} is a coherent analytic sheaf if:

  1. β„±\mathcal{F} is of finite type;
  2. for every open UβŠ†XU \subseteq X, every integer qβ‰₯0q \ge 0 and every morphism φ ⁣:π’ͺUqβ†’β„±βˆ£U\varphi \colon \mathcal{O}_U^q \to \mathcal{F}|_U of sheaves of modules, the kernel of Ο†\varphi is of finite type.

Remarks

Example: the sheaf π’ͺXp\mathcal{O}_X^p is coherent for every complex space XX; for X=β„‚nX = \C^n this is the Oka coherence theorem, and the general case follows from it. Given it, a sheaf is coherent if and only if it has, locally, a presentation π’ͺUqβ†’π’ͺUpβ†’β„±βˆ£Uβ†’0\mathcal{O}_U^q \to \mathcal{O}_U^p \to \mathcal{F}|_U \to 0.

Non-example: let X=β„‚X = \C, U=β„‚βˆ–{0}U = \C \setminus \set{0}, and let β„±\mathcal{F} be the sheaf of holomorphic functions on UU extended by zero to β„‚\C. Its stalk at 00 is 00 and its stalks at other points are π’ͺβ„‚,x\mathcal{O}_{\C,x}. A section of β„±\mathcal{F} on a connected open set Vβˆ‹0V \ni 0 is a holomorphic function on Vβˆ–{0}V \setminus \set{0} that vanishes near 00, so it is zero by the identity theorem. So no finite set of sections over a neighbourhood of 00 generates the nonzero stalks near 00, and clause 1 fails.

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  1. Complex analytic space concept
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Further reading

  1. Web https://en.wikipedia.org/wiki/Coherent_sheaf
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TagWKD2S2XA9C
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Kinddefinition
WritingW0 stub
MSC32C35
DefinesCoherent analytic sheaf
UsesComplex analytic space, Sheaf of modules

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