Affine variety

In algebraic geometry, an affine variety, or affine algebraic variety, over an algebraically closed field is the zero-locus in the affine space of some finite family of polynomials of variables with coefficients in that generate a prime ideal. If the condition of generating a prime ideal is removed, such a set is called an (affine) algebraic set. A Zariski open subvariety of an affine variety is called a quasi-affine variety.
Some texts do not require a prime ideal, and call "irreducible" an algebraic variety defined by a prime ideal. This article refers to zero-loci of not necessarily prime ideals as affine algebraic sets.
In some contexts, it is useful to distinguish the field in which the coefficients are considered, from the algebraically closed field (containing ) over which the zero-locus is considered (that is, the points of the affine variety are in ). In this case, the variety is said "defined over" , and the points of the variety that belong to are said "-rational" or "rational over" . In the common case where is the field of real numbers, a -rational point is called a "real point". When the field is not specified, a "rational point" is a point that is rational over the rational numbers. For example, Fermat's Last Theorem asserts that the affine algebraic variety (it is a curve) defined by has no rational points for any integer greater than two.
Introduction.
An affine algebraic set is the set of solutions in an algebraically closed field "k" of a system of polynomial equations with coefficients in "k". More precisely, if formula_1 are polynomials with coefficients in "k", they define an affine algebraic set 
An affine (algebraic) variety is an affine algebraic set which is not the union of two proper affine algebraic subsets. Such an affine algebraic set is often said to be "irreducible". 
If "X" is an affine algebraic set defined by an ideal "I", then the quotient ring formula_3 is called the coordinate ring of "X". If "X" is an affine variety, then "I" is prime, so the coordinate ring is an integral domain. The elements of the coordinate ring "R" are also called the "regular functions" or the "polynomial functions" on the variety. They form the "ring of regular functions" on the variety, or, simply, the "ring of the variety"; in other words (see #Structure sheaf), it is the space of global sections of the structure sheaf of "X".
The dimension of a variety is an integer associated to every variety, and even to every algebraic set, whose importance relies on the large number of its equivalent definitions (see Dimension of an algebraic variety).
Rational points.
For an affine variety formula_8 over an algebraically closed field , and a subfield of , a -"rational point" of is a point formula_9 That is, a point of whose coordinates are elements of . The collection of -rational points of an affine variety is often denoted formula_10 Often, if the base field is the complex numbers , points which are -rational (where is the real numbers) are called "real points" of the variety, and -rational points ( the rational numbers) are often simply called "rational points".
For instance, is a -rational and an -rational point of the variety formula_11 as it is in and all its coordinates are integers. The point is a real point of that is not -rational, and formula_12 is a point of that is not -rational. This variety is called a circle, because the set of its -rational points is the unit circle. It has infinitely many -rational points that are the points 
where is a rational number.
The circle formula_14 is an example of an algebraic curve of degree two that has no -rational point. This can be deduced from the fact that, modulo , the sum of two squares cannot be .
It can be proved that an algebraic curve of degree two with a -rational point has infinitely many other -rational points; each such point is the second intersection point of the curve and a line with a rational slope passing through the rational point.
The complex variety formula_15 has no -rational points, but has many complex points.
If is an affine variety in defined over the complex numbers , the -rational points of can be drawn on a piece of paper or by graphing software. The figure on the right shows the -rational points of formula_16
Singular points and tangent space.
Let be an affine variety defined by the polynomials formula_17 and formula_18 be a point of .
The Jacobian matrix of at is the matrix of the partial derivatives 
The point is "regular" if the rank of equals the dimension of , and "singular" otherwise.
If is regular, the tangent space to at is the affine subspace of formula_7 defined by the linear equations
If the point is singular, the affine subspace defined by these equations is also called a tangent space by some authors, while other authors say that there is no tangent space at a singular point.
A more intrinsic definition, which does not use coordinates is given by Zariski tangent space.
The Zariski topology.
The affine algebraic sets of "k""n" form the closed sets of a topology on "k""n", called the Zariski topology. This follows from the fact that formula_22 formula_23 formula_24 and formula_25 (in fact, a countable intersection of affine algebraic sets is an affine algebraic set).
The Zariski topology can also be described by way of basic open sets, where Zariski-open sets are countable unions of sets of the form formula_26 for formula_27 These basic open sets are the complements in "k""n" of the closed sets formula_28 zero loci of a single polynomial. If "k" is Noetherian (for instance, if "k" is a field or a principal ideal domain), then every ideal of "k" is finitely-generated, so every open set is a finite union of basic open sets.
If "V" is an affine subvariety of "k""n" the Zariski topology on "V" is simply the subspace topology inherited from the Zariski topology on "k""n".
Geometry–algebra correspondence.
The geometric structure of an affine variety is linked in a deep way to the algebraic structure of its coordinate ring. Let "I" and "J" be ideals of "k[V]", the coordinate ring of an affine variety "V". Let "I(V)" be the set of all polynomials in formula_29 which vanish on "V", and let formula_30 denote the radical of the ideal "I", the set of polynomials "f" for which some power of "f" is in "I". The reason that the base field is required to be algebraically closed is that affine varieties automatically satisfy Hilbert's nullstellensatz: for an ideal "J" in formula_29 where "k" is an algebraically closed field, formula_32
Radical ideals (ideals which are their own radical) of "k[V]" correspond to algebraic subsets of "V". Indeed, for radical ideals "I" and "J", formula_33 if and only if formula_34 Hence "V(I)=V(J)" if and only if "I=J". Furthermore, the function taking an affine algebraic set "W" and returning "I(W)", the set of all functions which also vanish on all points of "W", is the inverse of the function assigning an algebraic set to a radical ideal, by the nullstellensatz. Hence the correspondence between affine algebraic sets and radical ideals is a bijection. The coordinate ring of an affine algebraic set is reduced (nilpotent-free), as an ideal "I" in a ring "R" is radical if and only if the quotient ring "R/I" is reduced.
Prime ideals of the coordinate ring correspond to affine subvarieties. An affine algebraic set "V(I)" can be written as the union of two other algebraic sets if and only if "I=JK" for proper ideals "J" and "K" not equal to "I" (in which case formula_35). This is the case if and only if "I" is not prime. Affine subvarieties are precisely those whose coordinate ring is an integral domain. This is because an ideal is prime if and only if the quotient of the ring by the ideal is an integral domain.
Maximal ideals of "k[V]" correspond to points of "V". If "I" and "J" are radical ideals, then formula_36 if and only if formula_37 As maximal ideals are radical, maximal ideals correspond to minimal algebraic sets (those which contain no proper algebraic subsets), which are points in "V". If "V" is an affine variety with coordinate ring formula_38 this correspondence becomes explicit through the map formula_39 where formula_40 denotes the image in the quotient algebra "R" of the polynomial formula_41 An algebraic subset is a point if and only if the coordinate ring of the subset is a field, as the quotient of a ring by a maximal ideal is a field.
The following table summarises this correspondence, for algebraic subsets of an affine variety and ideals of the corresponding coordinate ring:
Products of affine varieties.
A product of affine varieties can be defined using the isomorphism then embedding the product in this new affine space. Let and have coordinate rings and respectively, so that their product has coordinate ring . Let be an algebraic subset of and an algebraic subset of Then each is a polynomial in , and each is in . The product of and is defined as the algebraic set in The product is irreducible if each , is irreducible.
It is important to note that the Zariski topology on is not the topological product of the Zariski topologies on the two spaces. Indeed, the product topology is generated by products of the basic open sets and Hence, polynomials that are in but not in or will define algebraic sets that are in the Zariski topology on but not in the product topology.
Morphisms of affine varieties.
A morphism, or regular map, of affine varieties is a function between affine varieties which is polynomial in each coordinate: more precisely, for affine varieties and , a morphism from to is a map of the form where for each These are the morphisms in the category of affine varieties.
There is a one-to-one correspondence between morphisms of affine varieties over an algebraically closed field and homomorphisms of coordinate rings of affine varieties over going in the opposite direction. Because of this, along with the fact that there is a one-to-one correspondence between affine varieties over and their coordinate rings, the category of affine varieties over is dual to the category of coordinate rings of affine varieties over The category of coordinate rings of affine varieties over is precisely the category of finitely-generated, nilpotent-free algebras over 
More precisely, for each morphism of affine varieties, there is a homomorphism between the coordinate rings (going in the opposite direction), and for each such homomorphism, there is a morphism of the varieties associated to the coordinate rings. This can be shown explicitly: let and be affine varieties with coordinate rings and respectively. Let be a morphism. Indeed, a homomorphism between polynomial rings factors uniquely through the ring and a homomorphism is determined uniquely by the images of Hence, each homomorphism corresponds uniquely to a choice of image for each . Then given any morphism from to a homomorphism can be constructed which sends to formula_42 where formula_43 is the equivalence class of in 
Similarly, for each homomorphism of the coordinate rings, a morphism of the affine varieties can be constructed in the opposite direction. Mirroring the paragraph above, a homomorphism sends to a polynomial formula_44 in . This corresponds to the morphism of varieties defined by 
Structure sheaf.
Equipped with the structure sheaf described below, an affine variety is a locally ringed space.
Given an affine variety "X" with coordinate ring "A", the sheaf of "k"-algebras formula_45 is defined by letting formula_46 be the ring of regular functions on "U".
Let "D"("f") = { "x" | "f"("x") ≠ 0 } for each "f" in "A". They form a base for the topology of "X" and so formula_45 is determined by its values on the open sets "D"("f"). (See also: sheaf of modules#Sheaf associated to a module.)
The key fact, which relies on Hilbert nullstellensatz in the essential way, is the following:
Proof: The inclusion ⊃ is clear. For the opposite, let "g" be in the left-hand side and formula_48, which is an ideal. If "x" is in "D"("f"), then, since "g" is regular near "x", there is some open affine neighborhood "D"("h") of "x" such that formula_49; that is, "h""m" "g" is in "A" and thus "x" is not in "V"("J"). In other words, formula_50 and thus the Hilbert nullstellensatz implies "f" is in the radical of "J"; i.e., formula_51. formula_52
The claim, first of all, implies that "X" is a "locally ringed" space since
where formula_54. Secondly, the claim implies that formula_45 is a sheaf; indeed, it says if a function is regular (pointwise) on "D"("f"), then it must be in the coordinate ring of "D"("f"); that is, "regular-ness" can be patched together.
Hence, formula_56 is a locally ringed space.
Serre's theorem on affineness.
A theorem of Serre gives a cohomological characterization of an affine variety; it says an algebraic variety is affine if and only if formula_57 for any formula_58 and any quasi-coherent sheaf "F" on "X". (cf. Cartan's theorem B.) This makes the cohomological study of an affine variety non-existent, in a sharp contrast to the projective case in which cohomology groups of line bundles are of central interest.
Affine algebraic groups.
An affine variety over an algebraically closed field is called an affine algebraic group if it has:
Together, these define a group structure on the variety. The above morphisms are often written using ordinary group notation: can be written as , or ; the inverse can be written as or Using the multiplicative notation, the associativity, identity and inverse laws can be rewritten as: , and .
The most prominent example of an affine algebraic group is the general linear group of degree This is the group of linear transformations of the vector space if a basis of is fixed, this is equivalent to the group of invertible matrices with entries in It can be shown that any affine algebraic group is isomorphic to a subgroup of . For this reason, affine algebraic groups are often called linear algebraic groups. 
Affine algebraic groups play an important role in the classification of finite simple groups, as the groups of Lie type are all sets of -rational points of an affine algebraic group, where is a finite field.
References.
The original article was written as a partial human translation of the corresponding French article.