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 ...

, a moduli scheme is a

moduli space In mathematics, in particular algebraic geometry, a moduli space is a geometric space (usually a scheme or an algebraic stack) whose points represent algebro-geometric objects of some fixed kind, or isomorphism classes of such objects. Such spac ...

that exists in the

category of schemes In mathematics, a scheme is a mathematical structure that enlarges the notion of algebraic variety in several ways, such as taking account of multiplicities (the equations ''x'' = 0 and ''x''2 = 0 define the same algebraic variety but different sch ...

developed by Alexander Grothendieck. Some important

moduli problem In mathematics, in particular algebraic geometry, a moduli space is a geometric space (usually a scheme or an algebraic stack) whose points represent algebro-geometric objects of some fixed kind, or isomorphism classes of such objects. Such s ...

s of

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 ...

can be satisfactorily solved by means of scheme theory alone, while others require some extension of the 'geometric object' concept ( algebraic spaces,

algebraic stack In mathematics, an algebraic stack is a vast generalization of algebraic spaces, or schemes, which are foundational for studying moduli theory. Many moduli spaces are constructed using techniques specific to algebraic stacks, such as Artin's repr ...

s of Michael Artin).

History

Work of Grothendieck and David Mumford (see geometric invariant theory) opened up this area in the early 1960s. The more algebraic and abstract approach to moduli problems is to set them up as a representable functor question, then apply a criterion that singles out the representable functors for schemes. When this programmatic approach works, the result is a ''fine moduli scheme''. Under the influence of more geometric ideas, it suffices to find a scheme that gives the correct geometric points. This is more like the classical idea that the moduli problem is to express the algebraic structure naturally coming with a set (say of isomorphism classes of elliptic curves). The result is then a ''coarse moduli scheme''. Its lack of refinement is, roughly speaking, that it doesn't guarantee for families of objects what is inherent in the fine moduli scheme. As Mumford pointed out in his book '' Geometric Invariant Theory'', one might want to have the fine version, but there is a technical issue ( level structure and other 'markings') that must be addressed to get a question with a chance of having such an answer. Teruhisa Matsusaka proved a result, now known as Matsusaka's big theorem, establishing a necessary condition on a

for the existence of a coarse moduli scheme.

Examples

Mumford proved that if ''g'' > 1, there exists a coarse moduli scheme of smooth curves of genus ''g'', which is quasi-projective. According to a recent survey by János Kollár, it "has a rich and intriguing intrinsic geometry which is related to major questions in many branches of mathematics and theoretical physics." Braungardt has posed the question whether Belyi's theorem can be generalised to varieties of higher dimension over the field of algebraic numbers, with the formulation that they are generally birational to a finite étale covering of a moduli space of curves. Using the notion of stable vector bundle, coarse moduli schemes for the vector bundles on any smooth complex variety have been shown to exist, and to be quasi-projective: the statement uses the concept of semistability. It is possible to identify the coarse moduli space of special instanton bundles, in mathematical physics, with objects in the classical geometry of conics, in certain cases.

References

Notes

Moduli theory Representable functors {{algebraic-geometry-stub