In
mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, algebraic geometry and analytic geometry are two closely related subjects. While
algebraic geometry
Algebraic geometry is a branch of mathematics, classically studying zeros of multivariate polynomials. Modern algebraic geometry is based on the use of abstract algebraic techniques, mainly from commutative algebra, for solving geometrical ...
studies
algebraic varieties
Algebraic varieties are the central objects of study in algebraic geometry, a sub-field of mathematics. Classically, an algebraic variety is defined as the set of solutions of a system of polynomial equations over the real or complex numbers. ...
, analytic geometry deals with
complex manifold
In differential geometry and complex geometry, a complex manifold is a manifold with an atlas of charts to the open unit disc in \mathbb^n, such that the transition maps are holomorphic.
The term complex manifold is variously used to mean a ...
s and the more general
analytic space
An analytic space is a generalization of an analytic manifold that allows singularities. An analytic space is a space that is locally the same as an analytic variety. They are prominent in the study of several complex variables, but they also ...
s defined locally by the vanishing of
analytic function
In mathematics, an analytic function is a function that is locally given by a convergent power series. There exist both real analytic functions and complex analytic functions. Functions of each type are infinitely differentiable, but complex ...
s of
several complex variables
The theory of functions of several complex variables is the branch of mathematics dealing with complex-valued functions. The name of the field dealing with the properties of function of several complex variables is called several complex variable ...
. The deep relation between these subjects has numerous applications in which algebraic techniques are applied to analytic spaces and analytic techniques to algebraic varieties.
Main statement
Let ''X'' be a projective complex
algebraic variety
Algebraic varieties are the central objects of study in algebraic geometry, a sub-field of mathematics. Classically, an algebraic variety is defined as the set of solutions of a system of polynomial equations over the real or complex numbers. ...
. Because ''X'' is a complex variety, its set of complex points ''X''(C) can be given the structure of a compact
complex analytic space. This analytic space is denoted ''X''
an. Similarly, if
is a sheaf on ''X'', then there is a corresponding sheaf
on ''X''
an. This association of an analytic object to an algebraic one is a functor. The prototypical theorem relating ''X'' and ''X''
an says that for any two
coherent sheaves and
on ''X'', the natural homomorphism:
:
is an isomorphism. Here
is the
structure sheaf
In mathematics, a ringed space is a family of ( commutative) rings parametrized by open subsets of a topological space together with ring homomorphisms that play roles of restrictions. Precisely, it is a topological space equipped with a sheaf ...
of the algebraic variety ''X'' and
is the structure sheaf of the analytic variety ''X''
an. In other words, the category of coherent sheaves on the algebraic variety ''X'' is equivalent to the category of analytic coherent sheaves on the analytic variety ''X''
an, and the equivalence is given on objects by mapping
to
. (Note in particular that
itself is coherent, a result known as the
Oka coherence theorem
In mathematics, the Oka coherence theorem, proved by , states that the sheaf \mathcal := \mathcal_ of germs of holomorphic functions on \mathbb^n over a complex manifold is coherent.In paper it was called the idéal de domaines indéterminé ...
.)
Another important statement is as follows: For any coherent sheaf
on an algebraic variety ''X'' the homomorphisms
:
are isomorphisms for all ''qs. This means that the ''q''-th cohomology group on ''X'' is isomorphic to the cohomology group on ''X''
an.
The theorem applies much more generally than stated above (see the
formal statement below). It and its proof have many consequences, such as
Chow's theorem, the
Lefschetz principle and
Kodaira vanishing theorem.
Background
Algebraic varieties are locally defined as the common zero sets of polynomials and since polynomials over the complex numbers are
holomorphic function
In mathematics, a holomorphic function is a complex-valued function of one or more complex variables that is complex differentiable in a neighbourhood of each point in a domain in complex coordinate space . The existence of a complex deriv ...
s, algebraic varieties over C can be interpreted as analytic spaces. Similarly, regular morphisms between varieties are interpreted as holomorphic mappings between analytic spaces. Somewhat surprisingly, it is often possible to go the other way, to interpret analytic objects in an algebraic way.
For example, it is easy to prove that the analytic functions from the
Riemann sphere
In mathematics, the Riemann sphere, named after Bernhard Riemann, is a model of the extended complex plane: the complex plane plus one point at infinity. This extended plane represents the extended complex numbers, that is, the complex numbers ...
to itself are either
the rational functions or the identically infinity function (an extension of
Liouville's theorem). For if such a function ''f'' is nonconstant, then since the set of ''z'' where ''f(z)'' is infinity is isolated and the Riemann sphere is compact, there are finitely many ''z'' with ''f(z)'' equal to infinity. Consider the
Laurent expansion
In mathematics, the Laurent series of a complex function f(z) is a representation of that function as a power series which includes terms of negative degree. It may be used to express complex functions in cases where a Taylor series expansio ...
at all such ''z'' and subtract off the singular part: we are left with a function on the Riemann sphere with values in C, which by Liouville's theorem is constant. Thus ''f'' is a rational function. This fact shows there is no essential difference between the
complex projective line as an algebraic variety, or as the
Riemann sphere
In mathematics, the Riemann sphere, named after Bernhard Riemann, is a model of the extended complex plane: the complex plane plus one point at infinity. This extended plane represents the extended complex numbers, that is, the complex numbers ...
.
Important results
There is a long history of comparison results between algebraic geometry and analytic geometry, beginning in the nineteenth century. Some of the more important advances are listed here in chronological order.
Riemann's existence theorem
Riemann surface
In mathematics, particularly in complex analysis, a Riemann surface is a connected one-dimensional complex manifold. These surfaces were first studied by and are named after Bernhard Riemann. Riemann surfaces can be thought of as deformed ver ...
theory shows that a
compact
Compact as used in politics may refer broadly to a pact or treaty; in more specific cases it may refer to:
* Interstate compact
* Blood compact, an ancient ritual of the Philippines
* Compact government, a type of colonial rule utilized in Britis ...
Riemann surface has enough
meromorphic function
In the mathematical field of complex analysis, a meromorphic function on an open subset ''D'' of the complex plane is a function that is holomorphic on all of ''D'' ''except'' for a set of isolated points, which are poles of the function. The ...
s on it, making it an
algebraic curve
In mathematics, an affine algebraic plane curve is the zero set of a polynomial in two variables. A projective algebraic plane curve is the zero set in a projective plane of a homogeneous polynomial in three variables. An affine algebraic plane ...
. Under the name Riemann's existence theorem
a deeper result on ramified coverings of a compact Riemann surface was known: such ''finite'' coverings as
topological space
In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called poin ...
s are classified by
permutation representation
In mathematics, the term permutation representation of a (typically finite) group G can refer to either of two closely related notions: a representation of G as a group of permutations, or as a group of permutation matrices. The term also refers ...
s of the
fundamental group
In the mathematical field of algebraic topology, the fundamental group of a topological space is the group of the equivalence classes under homotopy of the loops contained in the space. It records information about the basic shape, or holes, o ...
of the complement of the
ramification point
In geometry, ramification is 'branching out', in the way that the square root function, for complex numbers, can be seen to have two ''branches'' differing in sign. The term is also used from the opposite perspective (branches coming together) as ...
s. Since the Riemann surface property is local, such coverings are quite easily seen to be coverings in the complex-analytic sense. It is then possible to conclude that they come from covering maps of algebraic curves—that is, such coverings all come from
finite extensions of the
function field.
The Lefschetz principle
In the twentieth century, the Lefschetz principle, named for
Solomon Lefschetz, was cited in algebraic geometry to justify the use of topological techniques for algebraic geometry over any
algebraically closed field
In mathematics, a field is algebraically closed if every non-constant polynomial in (the univariate polynomial ring with coefficients in ) has a root in .
Examples
As an example, the field of real numbers is not algebraically closed, because ...
''K'' of
characteristic 0, by treating ''K'' as if it were the complex number field. An elementary form of it asserts that true statements of the first order theory of fields about C are true for any algebraically closed field ''K'' of characteristic zero. A precise principle and its proof are due to
Alfred Tarski
Alfred Tarski (, born Alfred Teitelbaum;School of Mathematics and Statistics, University of St Andrews ''School of Mathematics and Statistics, University of St Andrews''. January 14, 1901 – October 26, 1983) was a Polish-American logician a ...
and are based in
mathematical logic
Mathematical logic is the study of formal logic within mathematics. Major subareas include model theory, proof theory, set theory, and recursion theory. Research in mathematical logic commonly addresses the mathematical properties of forma ...
.
This principle permits the carrying over of some results obtained using analytic or topological methods for algebraic varieties over C to other algebraically closed ground fields of characteristic 0.
Chow's theorem
, proved by
Wei-Liang Chow, is an example of the most immediately useful kind of comparison available. It states that an analytic subspace of complex
projective space
In mathematics, the concept of a projective space originated from the visual effect of perspective, where parallel lines seem to meet ''at infinity''. A projective space may thus be viewed as the extension of a Euclidean space, or, more generally ...
that is closed (in the ordinary topological sense) is an algebraic subvariety. This can be rephrased as "any analytic subspace of complex projective space which is closed in the
strong topology is closed in the
Zariski topology
In algebraic geometry and commutative algebra, the Zariski topology is a topology which is primarily defined by its closed sets. It is very different from topologies which are commonly used in the real or complex analysis; in particular, it is n ...
." This allows quite a free use of complex-analytic methods within the classical parts of algebraic geometry.
GAGA
Foundations for the many relations between the two theories were put in place during the early part of the 1950s, as part of the business of laying the foundations of algebraic geometry to include, for example, techniques from
Hodge theory
In mathematics, Hodge theory, named after W. V. D. Hodge, is a method for studying the cohomology groups of a smooth manifold ''M'' using partial differential equations. The key observation is that, given a Riemannian metric on ''M'', every co ...
. The major paper consolidating the theory was ''Géometrie Algébrique et Géométrie Analytique'' by
Jean-Pierre Serre
Jean-Pierre Serre (; born 15 September 1926) is a French mathematician who has made contributions to algebraic topology, algebraic geometry, and algebraic number theory. He was awarded the Fields Medal in 1954, the Wolf Prize in 2000 and the ina ...
, now usually referred to as GAGA. It proves general results that relate classes of algebraic varieties, regular morphisms and
sheaves with classes of analytic spaces, holomorphic mappings and sheaves. It reduces all of these to the comparison of categories of sheaves.
Nowadays the phrase ''GAGA-style result'' is used for any theorem of comparison, allowing passage between a category of objects from algebraic geometry, and their morphisms, to a well-defined subcategory of analytic geometry objects and holomorphic mappings.
Formal statement of GAGA
# Let
be a scheme of finite type over C. Then there is a topological space ''X''
an which as a set consists of the closed points of ''X'' with a continuous inclusion map λ
X: ''X''
an → ''X''. The topology on ''X''
an is called the "complex topology" (and is very different from the subspace topology).
# Suppose φ: ''X'' → ''Y'' is a morphism of schemes of locally finite type over C. Then there exists a continuous map φ
an: ''X''
an → ''Y''
an such λ
''Y'' ° φ
an = φ ° λ
X.
# There is a sheaf
on ''X''
an such that
is a ringed space and λ
X: ''X''
an → ''X'' becomes a map of ringed spaces. The space
is called the "analytification" of
and is an analytic space. For every φ: ''X'' → ''Y'' the map φ
an defined above is a mapping of analytic spaces. Furthermore, the map φ ↦ φ
an maps open immersions into open immersions. If ''X'' = ''Spec''(C
1,...,''x''n">'x''1,...,''x''n then ''X''
an = C
''n'' and
for every polydisc ''U'' is a suitable quotient of the space of holomorphic functions on ''U''.
# For every sheaf
on ''X'' (called algebraic sheaf) there is a sheaf
on ''X''
an (called analytic sheaf) and a map of sheaves of
-modules
. The sheaf
is defined as
. The correspondence
defines an exact functor from the category of sheaves over
to the category of sheaves of
.
The following two statements are the heart of Serre's GAGA theorem (as extended by
Alexander Grothendieck,
Amnon Neeman,
and others.)
# If ''f'': ''X'' → ''Y'' is an arbitrary morphism of schemes of finite type over C and
is coherent then the natural map
is injective. If ''f'' is proper then this map is an isomorphism. One also has isomorphisms of all higher direct image sheaves
in this case.
# Now assume that ''X''
an is Hausdorff and compact. If
are two coherent algebraic sheaves on
and if
is a map of sheaves of
-modules then there exists a unique map of sheaves of
-modules
with
. If
is a coherent analytic sheaf of
-modules over ''X''
an then there exists a coherent algebraic sheaf
of
-modules and an isomorphism
.
In slightly lesser generality, the GAGA theorem asserts that the category of coherent algebraic sheaves on a complex projective variety ''X'' and the category of coherent analytic sheaves on the corresponding analytic space ''X''
an are equivalent. The analytic space ''X''
an is obtained roughly by pulling back to ''X'' the complex structure from C
n through the coordinate charts. Indeed, phrasing the theorem in this manner is closer in spirit to Serre's paper, seeing how the full scheme-theoretic language that the above formal statement uses heavily had not yet been invented by the time of GAGA's publication.
Notes
References
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External links
* Kiran Kedlaya. 18.72
Algebraic GeometryLEC # 30 - 33 GAGASpring 2009. Massachusetts Institute of Technology: MIT OpenCourseWare Creative Commons
BY-NC-SA
A Creative Commons (CC) license is one of several public copyright licenses that enable the free distribution of an otherwise copyrighted "work".A "work" is any creative material made by a person. A painting, a graphic, a book, a song/lyric ...
.
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