Complex analytic variety

Generalization of a complex manifold that allows the use of singularities

In mathematics, particularly differential geometry and complex geometry, a complex analytic variety[note 1] or complex analytic space is a generalization of a complex manifold that allows the presence of singularities. Complex analytic varieties are locally ringed spaces that are locally isomorphic to local model spaces, where a local model space is an open subset of the vanishing locus of a finite set of holomorphic functions.

## Definition

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Denote the constant sheaf on a topological space with value  C {\displaystyle \mathbb {C} } by  C _ {\displaystyle {\underline {\mathbb {C} }}} . A  C {\displaystyle \mathbb {C} } -space is a locally ringed space ( X , O X ) {\displaystyle (X,{\mathcal {O}}_{X})} , whose structure sheaf is an algebra over  C _ {\displaystyle {\underline {\mathbb {C} }}} .

Choose an open subset  U {\displaystyle U} of some complex affine space C n {\displaystyle \mathbb {C} ^{n}} , and fix finitely many holomorphic functions  f 1 , … , f k {\displaystyle f_{1},\dots ,f_{k}} in  U {\displaystyle U} . Let  X = V ( f 1 , … , f k ) {\displaystyle X=V(f_{1},\dots ,f_{k})} be the common vanishing locus of these holomorphic functions, that is,  X = { x ∣ f 1 ( x ) = ⋯ = f k ( x ) = 0 } {\displaystyle X=\\{x\mid f_{1}(x)=\cdots =f_{k}(x)=0\\}} . Define a sheaf of rings on  X {\displaystyle X} by letting  O X {\displaystyle {\mathcal {O}}_{X}} be the restriction to  X {\displaystyle X} of  O U / ( f 1 , … , f k ) {\displaystyle {\mathcal {O}}_{U}/(f_{1},\ldots ,f_{k})} , where  O U {\displaystyle {\mathcal {O}}_{U}} is the sheaf of holomorphic functions on  U {\displaystyle U} . Then the locally ringed  C {\displaystyle \mathbb {C} } -space  ( X , O X ) {\displaystyle (X,{\mathcal {O}}_{X})} is a local model space.

A complex analytic variety is a locally ringed  C {\displaystyle \mathbb {C} } -space  ( X , O X ) {\displaystyle (X,{\mathcal {O}}_{X})} that is locally isomorphic to a local model space.

Morphisms of complex analytic varieties are defined to be morphisms of the underlying locally ringed spaces, they are also called holomorphic maps. A structure sheaf may have nilpotent element,[1] and also, when the complex analytic space whose structure sheaf is reduced, then the complex analytic space is reduced, that is, the complex analytic space may not be reduced.

An associated complex analytic space (variety) X h {\displaystyle X_{h}} is such that;[1]

Let X be scheme of finite type over  C {\displaystyle \mathbb {C} } , and cover X with open affine subsets  Y i = Spec ⁡ A i {\displaystyle Y_{i}=\operatorname {Spec} A_{i}} ( X = ∪ Y i {\displaystyle X=\cup Y_{i}} ) (Spectrum of a ring). Then each  A i {\displaystyle A_{i}} is an algebra of finite type over  C {\displaystyle \mathbb {C} } , and  A i ≃ C [ z 1 , … , z n ] / ( f 1 , … , f m ) {\displaystyle A_{i}\simeq \mathbb {C} [z_{1},\dots ,z_{n}]/(f_{1},\dots ,f_{m})} . Where  f 1 , … , f m {\displaystyle f_{1},\dots ,f_{m}} are polynomial in  z 1 , … , z n {\displaystyle z_{1},\dots ,z_{n}} , which can be regarded as a holomorphic functions on  C {\displaystyle \mathbb {C} } . Therefore, their set of common zeros is the complex analytic subspace  ( Y i ) h ⊆ C {\displaystyle (Y_{i})_{h}\subseteq \mathbb {C} } . Here, the scheme X obtained by glueing the data of the sets  Y i {\displaystyle Y_{i}} , and then the same data can be used for glueing the complex analytic spaces  ( Y i ) h {\displaystyle (Y_{i})_{h}} into a complex analytic space  X h {\displaystyle X_{h}} , so we call  X h {\displaystyle X_{h}} an associated complex analytic space with X. The complex analytic space X is reduced if and only if the associated complex analytic space  X h {\displaystyle X_{h}} is reduced.[2]

## See also

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* Algebraic variety \- Roughly speaking, an (complex) analytic variety is a zero locus of a set of an (complex) analytic function, while an algebraic variety is a zero locus of a set of a polynomial function and allowing singular point.
* Analytic space – locally ringed space glued together from analytic varietiesPages displaying wikidata descriptions as a fallback
* Complex algebraic variety
* GAGA – Two closely related mathematical subjectsPages displaying short descriptions of redirect targets
* Rigid analytic space – Analogue of a complex analytic space over a nonarchimedean field

## Note

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1. ^ a b Hartshorne 1977, p. 439.
2. ^ Grothendieck & Raynaud (2002) (SGA 1 §XII. Proposition 2.1.)

## Annotation

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1. ^ Complex analytic variety (or just variety) is sometimes required to be irreducible and (or) reduced

## References

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* Aroca, José Manuel; Hironaka, Heisuke; Vicente, José Luis (3 November 2018). Complex Analytic Desingularization. doi:10.1007/978-4-431-49822-3. ISBN 978-4-431-49822-3.
* Bloom, Thomas; Herrera, Miguel (1969). "De Rham cohomology of an analytic space". Inventiones Mathematicae. 7 (4): 275–296\. Bibcode:1969InMat...7..275B. doi:10.1007/BF01425536. S2CID 122113902.
* Cartan, H.; Bruhat, F.; Cerf, Jean.; Dolbeault, P.; Frenkel, Jean.; Hervé, Michel; Malatian.; Serre, J-P. "Séminaire Henri Cartan, Tome 4 (1951-1952)". (no.10-13)
* Fischer, G. (14 November 2006). Complex Analytic Geometry. Springer. ISBN 978-3-540-38121-1.
* Gunning, Robert Clifford; Rossi, Hugo (2009). "Chapter III. Variety (Sec. B. Analytic covers)". Analytic Functions of Several Complex Variables. American Mathematical Soc. ISBN 9780821821657.
* Gunning, Robert Clifford; Rossi, Hugo (2009). "Chapter V. Analytic spaces". Analytic Functions of Several Complex Variables. American Mathematical Soc. ISBN 9780821821657.
* Grauert, Hans; Remmert, Reinhold (1958). "Komplexe Räume". Mathematische Annalen. 136 (3): 245–318\. doi:10.1007/BF01362011. S2CID 121348794.
* Grauert, H.; Remmert, R. (6 December 2012). Coherent Analytic Sheaves. Springer. ISBN 978-3-642-69582-7.
* Grauert, H.; Peternell, Thomas; Remmert, R. (9 March 2013). Several Complex Variables VII: Sheaf-Theoretical Methods in Complex Analysis. Springer. ISBN 978-3-662-09873-8.
* Grothendieck, Alexander; Raynaud, Michèle (2002). "Revêtements étales et groupe fondamental§XII. Géométrie algébrique et géométrie analytique". Revêtements étales et groupe fondamental (SGA 1) (in French). arXiv:math/0206203. doi:10.1007/BFb0058656. ISBN 978-2-85629-141-2.
* Hartshorne, Robin (1970). Ample Subvarieties of Algebraic Varieties. Lecture Notes in Mathematics. Vol. 156\. doi:10.1007/BFb0067839. ISBN 978-3-540-05184-8.
* Hartshorne, Robin (1977). Algebraic Geometry. Graduate Texts in Mathematics. Vol. 52\. Berlin, New York: Springer-Verlag. doi:10.1007/978-1-4757-3849-0. ISBN 978-0-387-90244-9. MR 0463157. S2CID 197660097. Zbl 0367.14001.
* Huckleberry, Alan (2013). "Hans Grauert (1930–2011)". Jahresbericht der Deutschen Mathematiker-Vereinigung. 115: 21–45\. arXiv:1303.6933. doi:10.1365/s13291-013-0061-7. S2CID 119685542.
* Remmert, Reinhold (1998). "From Riemann Surfaces to Complex Spaces". Séminaires et Congrès. Zbl 1044.01520.
* Serre, Jean-Pierre (1956). "Géométrie algébrique et géométrie analytique". Annales de l'Institut Fourier. 6: 1–42\. doi:10.5802/aif.59. ISSN 0373-0956. MR 0082175.
* Tognoli, A. (2 June 2011). Tognoli, A (ed.). Singularities of Analytic Spaces: Lectures given at a Summer School of the Centro Internazionale Matematico Estivo (C.I.M.E.) held in Bressanone (Bolzano), Italy, June 16-25, 1974. doi:10.1007/978-3-642-10944-7. ISBN 978-3-642-10944-7.
* "Chapter II. Preliminaries". Zariski-decomposition and Abundance. Mathematical Society of Japan Memoirs. Vol. 14\. Mathematical Society of Japan. 2004. pp. 13–78\. doi:10.2969/msjmemoirs/01401C020. ISBN 978-4-931469-31-0.
* Flores, Arturo Giles; Teissier, Bernard (2018). "Local polar varieties in the geometric study of singularities". Annales de la Faculté des Sciences de Toulouse: Mathématiques. 27 (4): 679–775\. arXiv:1607.07979. doi:10.5802/afst.1582. S2CID 119150240.

## Future reading

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* Huckleberry, Alan (2013). "Hans Grauert (1930–2011)". Jahresbericht der Deutschen Mathematiker-Vereinigung. 115: 21–45\. doi:10.1365/s13291-013-0061-7. S2CID 256084531.

## External links

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* Kiran Kedlaya. 18.726 Algebraic Geometry (LEC # 30 - 33 GAGA)Spring 2009. Massachusetts Institute of Technology: MIT OpenCourseWare Creative Commons BY-NC-SA.
* Tasty Bits of Several Complex Variables (p. 137) open source book by Jiří Lebl BY-NC-SA.
* Onishchik, A.L. (2001) [1994], "Analytic space", Encyclopedia of Mathematics, EMS Press
* El'kin, A.G. (2001) [1994], "Analytic set", Encyclopedia of Mathematics, EMS Press