Schubert calculus

In mathematics, Schubert calculus[1] is a branch of algebraic geometry introduced in the nineteenth century by Hermann Schubert, in order to solve various counting problems of projective geometry (part of enumerative geometry). It was a precursor of several more modern theories, for example characteristic classes, and in particular its algorithmic aspects are still of current interest. The term Schubert calculus is sometimes used to mean the enumerative geometry of linear subspaces, roughly equivalent to describing the cohomology ring of Grassmannians, and sometimes to mean the more general enumerative geometry of algebraic varieties that are homogenous spaces of simple Lie groups. Even more generally, Schubert calculus is often understood to encompass the study of analogous questions in generalized cohomology theories.

The objects introduced by Schubert are the Schubert cells[2], which are locally closed sets in a Grassmannian defined by conditions of incidence of a linear subspace in projective space with a given flag. For further details see Schubert variety.

The intersection theory[3] of these cells, which can be seen as the product structure in the cohomology ring of the Grassmannian of associated cohomology classes, in principle allows the prediction of the cases where intersections of cells results in a finite set of points, which are potentially concrete answers to enumerative questions. A key result is that the Schubert cells (or rather, the classes of their Zariski closures, the Schubert cycles or Schubert varieties) span the whole cohomology ring.

The combinatorial aspects mainly arise in relation to computing intersections of Schubert cycles. Lifted from the Grassmannian, which is a homogeneous space, to the general linear group that acts on it, similar questions are involved in the Bruhat decomposition and classification of parabolic subgroups (as block traingular matrices).

Putting Schubert's system on a rigorous footing was Hilbert's fifteenth problem.

Construction

Schubert calculus can be constructed using the Chow ring [3] of the Grassmannian, where the generating cycles are represented by geometrically defined data.[4] Denote the Grassmannian of -planes in a fixed -dimensional vector space as , and its Chow ring as . Note that the Grassmannian is sometimes denoted if the vector space isn't explicitly given or as if the ambient space and its -dimensional subspaces are replaced by their projectizations. Choosing an (arbitrary) complete flag

to each weakly decreasing -tuple of integers , where

i.e., to each partition of weight

whose Young diagram fits into the rectangular one , we associate a Schubert variety[1][2] (or Schubert cycle) , defined as

This is the closure, in the Zariski topology, of the Schubert cell[1][2]

which is used when considering cellular homology instead of the Chow ring. The latter are disjoint affine spaces, of dimension , whose union is .

Since the homology class , called a Schubert class, does not depend on the choice of complete flag , it can be written as

It can be shown that these classes are linearly independent and generate the Chow ring as their linear span. The associated intersection theory is called Schubert calculus. For a given sequence with the Schubert class is usually just denoted . The Schubert classes given by a single integer , (i.e., a horizontal partition), are called special classes. Using the Giambelli formula below, all the Schubert classes can be generated from these special classes.

Other notational conventions

In some sources[1][2], the Schubert cells and Schubert varieties are labelled differently, as and , respectively, where is the complementary partition, to , with parts

,

whose Young diagram is the complement of the one for within the rectangular one (reversed, both horizontally and vertically).

Another labelling convention for and is and , respectively, where is the multi-index defined by

or, alternatively, by

.

Explanation

In order to explain the definition, consider a generic -plane . It will have only a zero intersection with for , whereas

for

For example, in , a -plane is the solution space of a system of five independent homogeneous linear equations. These equations will generically span when restricted to a subspace with , in which case the solution space (the intersection of with ) will consist only of the zero vector. However, if , and will necessarily have nonzero intersection. For example, the expected dimension of intersection of and is , the intersection of and has expected dimension , and so on.

The definition of a Schubert variety states that the first value of with is generically smaller than the expected value by the parameter . The -planes given by these constraints then define special subvarieties of .[4]

Inclusion

There is a partial ordering on all -tuples where if for every . This gives the inclusion of Schubert varieties

showing an increase of the indices corresponds to an even greater specialization of subvarieties.

Codimension formula

A Schubert variety has codimension , which is stable under inclusions of Grassmannians. That is, the inclusion

given by adding the extra basis element to each -plane, giving a -plane, has the property

Also, the inclusion

given by inclusion of the -plane has the same pullback property.

Intersection product

The intersection product was first established using the Pieri and Giambelli formulas.

Pieri formula

In the special case , there is an explicit formula of the product of with an arbitrary Schubert class given by

Note . This is called the Pieri formula, and can be used to determine the intersection product of any two Schubert classes when combined with the Giambelli formula. For example

and

Giambelli formula

Schubert classes for partitions of any length can be expressed as the determinant of a matrix having the special classes as entries.

This is known as the Giambelli formula. It has the same form as the first Jacobi-Trudi identity, expressing arbitrary Schur functions as determinants in terms of the complete symmetric functions .

For example,

and

General case

The intersection product between any pair of Schubert classes is given by

where are the Littlewood-Richardson coefficients.[5] The Pieri formula is a special case of this, when has length .

Relation with Chern classes

There is an easy description of the cohomology ring, or the Chow ring, of the Grassmannian using the Chern classes of two natural vector bundles over . We have the exact sequence of vector bundles over

where is the tautological bundle whose fiber, over any element is the subspace itself, is the trivial vector bundle of rank , with as fiber and is the quotient vector bundle of rank , with as fiber. The Chern classes of the bundles and are

where is the partition whose Young diagram consists of a single column of length and

The tautological sequence then gives the presentation of the Chow ring as

One of the classical examples analyzed is the Grassmannian since it parameterizes lines in . Using the Chow ring , Schubert calculus can be used to compute the number of lines on a cubic surface Cubic surface.[4]

Chow ring

The Chow ring has the presentation

and as a graded Abelian group[6] it is given by

Lines on a cubic surface

Recall that a line in gives a dimension two subspace of , hence an element of . Also, the equation of a line can be given as a section of . Since a cubic surface is given as a generic homogeneous cubic polynomial, this is given as a generic section . A line is a subvariety of if and only if the section vanishes on . Therefore, the Euler class of can be integrated over to get the number of points where the generic section vanishes on . In order to get the Euler class, the total Chern class of must be computed, which is given as

The splitting formula then reads as the formal equation

where and for formal line bundles . The splitting equation gives the relations

and .

Since can be viewed as the direct sum of formal line bundles

whose total Chern class is

it follows that

using the fact that

and

Since is the top class, the integral is then

Therefore there are lines on a cubic surface.

See also

References

  1. Kleiman, S.L.; Laksov, Dan (1972). "Schubert Calculus". American Mathematical Monthly. American Mathematical Society. 79 (10): 1061–1082. doi:10.1080/00029890.1972.11993188. ISSN 0377-9017.
  2. Fulton, William (1997). Young Tableaux. With Applications to Representation Theory and Geometry, Chapt. 9.4. London Mathematical Society Student Texts. Vol. 35. Cambridge, U.K.: Cambridge University Press. doi:10.1017/CBO9780511626241. ISBN 9780521567244.
  3. Fulton, William (1998). Intersection Theory. Berlin, New York: Springer-Verlag. ISBN 978-0-387-98549-7. MR 1644323.
  4. 3264 and All That (PDF). pp. 132, section 4.1, 200, section 6.2.1.
  5. Fulton, William (1997). Young Tableaux. With Applications to Representation Theory and Geometry, Chapt. 5. London Mathematical Society Student Texts. Vol. 35. Cambridge, U.K.: Cambridge University Press. doi:10.1017/CBO9780511626241. ISBN 9780521567244.
  6. Katz, Sheldon. Enumerative Geometry and String Theory. p. 96.
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