Spectrum of a ring

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In abstract algebra and algebraic geometry, the spectrum of a commutative ring R, denoted by Spec(R), is the set of all prime ideals of R. It is commonly augmented with the Zariski topology and with a structure sheaf, turning it into a locally ringed space.

Zariski topology

For any ideal I of R, define $V_{I}$ to be the set of prime ideals containing I. We can put a topology on Spec(R) by defining the collection of closed sets to be

$\{V_{I}\colon I{\text{ is an ideal of }}R\}.$ This topology is called the Zariski topology.

A basis for the Zariski topology can be constructed as follows. For fR, define Df to be the set of prime ideals of R not containing f. Then each Df is an open subset of Spec(R), and $\{D_{f}:f\in R\}$ is a basis for the Zariski topology.

Spec(R) is a compact space, but almost never Hausdorff: in fact, the maximal ideals in R are precisely the closed points in this topology. However, Spec(R) is always a Kolmogorov space. It is also a spectral space.

Sheaves and schemes

Given the space X=Spec(R) with the Zariski topology, the structure sheaf OX is defined on the Df by setting Γ(Df, OX) = Rf, the localization of R at the multiplicative system {1,f,f2,f3,...}. It can be shown that this satisfies the necessary axioms to be a B-Sheaf. Next, if U is the union of {Dfi}iI, we let Γ(U,OX) = limiI Rfi, and this produces a sheaf; see the Gluing axiom article for more detail.

If R is an integral domain, with field of fractions K, then we can describe the ring Γ(U,OX) more concretely as follows. We say that an element f in K is regular at a point P in X if it can be represented as a fraction f = a/b with b not in P. Note that this agrees with the notion of a regular function in algebraic geometry. Using this definition, we can describe Γ(U,OX) as precisely the set of elements of K which are regular at every point P in U.

If P is a point in Spec(R), that is, a prime ideal, then the stalk at P equals the localization of R at P, and this is a local ring. Consequently, Spec(R) is a locally ringed space.

Every locally ringed space isomorphic to one of this form is called an affine scheme. General schemes are obtained by "gluing together" several affine schemes.

Functoriality

It is useful to use the language of category theory and observe that Spec is a functor. Every ring homomorphism f : RS induces a continuous map Spec(f) : Spec(S) → Spec(R) (since the preimage of any prime ideal in S is a prime ideal in R). In this way, Spec can be seen as a contravariant functor from the category of commutative rings to the category of topological spaces. Moreover for every prime P the homomorphism f descends to homomorphisms

Of -1(P)OP

of local rings. Thus Spec even defines a contravariant functor from the category of commutative rings to the category of locally ringed spaces. In fact it is the universal such functor and this can be used to define the functor Spec up to natural isomorphism.

The functor Spec yields a contravariant equivalence between the category of commutative rings and the category of affine schemes; each of these categories is often thought of as the opposite category of the other.

Motivation from algebraic geometry

Following on from the example, in algebraic geometry one studies algebraic sets, i.e. subsets of Kn (where K is an algebraically closed field) that are defined as the common zeros of a set of polynomials in n variables. If A is such an algebraic set, one considers the commutative ring R of all polynomial functions AK. The maximal ideals of R correspond to the points of A (because K is algebraically closed), and the prime ideals of R correspond to the subvarieties of A (an algebraic set is called irreducible or a variety if it cannot be written as the union of two proper algebraic subsets).

The spectrum of R therefore consists of the points of A together with elements for all subvarieties of A. The points of A are closed in the spectrum, while the elements corresponding to subvarieties have a closure consisting of all their points and subvarieties. If one only considers the points of A, i.e. the maximal ideals in R, then the Zariski topology defined above coincides with the Zariski topology defined on algebraic sets (which has precisely the algebraic subsets as closed sets).

One can thus view the topological space Spec(R) as an "enrichment" of the topological space A (with Zariski topology): for every subvariety of A, one additional non-closed point has been introduced, and this point "keeps track" of the corresponding subvariety. One thinks of this point as the generic point for the subvariety. Furthermore, the sheaf on Spec(R) and the sheaf of polynomial functions on A are essentially identical. By studying spectra of polynomial rings instead of algebraic sets with Zariski topology, one can generalize the concepts of algebraic geometry to non-algebraically closed fields and beyond, eventually arriving at the language of schemes.

Global Spec

There is a relative version of the functor Spec called global Spec, or relative Spec, and denoted by Spec. For a scheme Y, and a quasi-coherent sheaf of OY-algebras A, there is a unique scheme SpecA, and a morphism $f\colon {\mathbf {Spec} }\ A\to Y$ such that for every open affine $U\subseteq Y$ , there is an isomorphism induced by f: $f^{-1}(U)\cong {\mathrm {Spec} }\ A(U)$ , and such that for open affines $U\subseteq V$ , the inclusion $f^{-1}(U)\to f^{-1}(V)$ induces the restriction map $A(V)\to A(U).$ That is, as ring homomorphisms induce opposite maps of spectra, the restriction maps of a sheaf of algebras induce the inclusion maps of the spectra that make up the Spec of the sheaf.

Representation theory perspective

From the perspective of representation theory, a prime ideal I corresponds to a module R/I, and the spectrum of a ring corresponds to irreducible cyclic representations of R, while more general subvarieties correspond to possibly reducible representations that need not be cyclic. Recall that abstractly, the representation theory of a group is the study of modules over its group algebra.

The connection to representation theory is clearer if one considers the polynomial ring $R=K[x_{1},\dots ,x_{n}]$ or, without a basis, $R=K[V].$ As the latter formulation makes clear, a polynomial ring is the group algebra over a vector space, and writing in terms of $x_{i}$ corresponds to choosing a basis for the vector space. Then an ideal I, or equivalently a module $R/I,$ is a cyclic representation of R (cyclic meaning generated by 1 element as an R-module; this generalizes 1-dimensional representations).

In the case that the field is algebraically closed (say, the complex numbers), every maximal ideal corresponds to a point in n-space, by the nullstellensatz (the maximal ideal generated by $(x_{1}-a_{1}),(x_{2}-a_{2}),\ldots ,(x_{n}-a_{n})$ corresponds to the point $(a_{1},\ldots ,a_{n})$ ). These representations of $K[V]$ are then parametrized by the dual space $V^{*},$ the covector being given by sending each $x_{i}$ to the corresponding $a_{i}$ . Thus a representation of $K^{n}$ (K-linear maps $K^{n}\to K$ ) is given by a set of n numbers, or equivalently a covector $K^{n}\to K.$ Thus, points in n-space, thought of as the max spec of $R=K[x_{1},\dots ,x_{n}],$ correspond precisely to 1-dimensional representations of R, while finite sets of points correspond to finite-dimensional representations (which are reducible, corresponding geometrically to being a union, and algebraically to not being a prime ideal). The non-maximal ideals then correspond to infinite-dimensional representations.

Functional analysis perspective

The term "spectrum" comes from the use in operator theory. Given a linear operator T on a finite-dimensional vector space V, one can consider the vector space with operator as a module over the polynomial ring in one variable R=K[T], as in the structure theorem for finitely generated modules over a principal ideal domain. Then the spectrum of K[T] (as a ring) equals the spectrum of T (as an operator).

Further, the geometric structure of the spectrum of the ring (equivalently, the algebraic structure of the module) captures the behavior of the spectrum of the operator, such as algebraic multiplicity and geometric multiplicity. For instance, for the 2×2 identity matrix has corresponding module:

$K[T]/(T-1)\oplus K[T]/(T-1)$ the 2×2 zero matrix has module

$K[T]/(T-0)\oplus K[T]/(T-0),$ showing geometric multiplicity 2 for the zero eigenvalue, while a non-trivial 2×2 nilpotent matrix has module

$K[T]/T^{2},$ showing algebraic multiplicity 2 but geometric multiplicity 1.

In more detail:

• the eigenvalues (with geometric multiplicity) of the operator correspond to the (reduced) points of the variety, with multiplicity;
• the primary decomposition of the module corresponds to the unreduced points of the variety;
• a diagonalizable (semisimple) operator corresponds to a reduced variety;
• a cyclic module (one generator) corresponds to the operator having a cyclic vector (a vector whose orbit under T spans the space);
• the last invariant factor of the module equals the minimal polynomial of the operator, and the product of the invariant factors equals the characteristic polynomial.

Generalizations

The spectrum can be generalized from rings to C*-algebras in operator theory, yielding the notion of the spectrum of a C*-algebra. Notably, for a Hausdorff space, the algebra of scalars (the bounded continuous functions on the space, being analogous to regular functions) are a commutative C*-algebra, with the space being recovered as a topological space from MSpec of the algebra of scalars, indeed functorially so; this is the content of the Banach–Stone theorem. Indeed, any commutative C*-algebra can be realized as the algebra of scalars of a Hausdorff space in this way, yielding the same correspondence as between a ring and its spectrum. Generalizing to non-commutative C*-algebras yields noncommutative topology.