Grassmannian

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In mathematics, the Grassmannian Gr(r, V) is a space which parameterizes all linear subspaces of a vector space Template:Mvar of given dimension Template:Mvar. For example, the Grassmannian Gr(1, V) is the space of lines through the origin in Template:Mvar, so it is the same as the projective space of one dimension lower than Template:Mvar.

When Template:Mvar is a real or complex vector space, Grassmannians are compact smooth manifolds.[1] In general they have the structure of a smooth algebraic variety.

The earliest work on a non-trivial Grassmannian is due to Julius Plücker, who studied the set of lines in projective 3-space and parameterized them by what are now called Plücker coordinates. Grassmannians are named after Hermann Grassmann, who introduced the concept in general.

Notations vary between authors, with Gr(V, r) being equivalent to Gr(r, V), and with some authors using Gr(r, n) or Gr(n, r) to denote the Grassmannian of Template:Mvar-dimensional subspaces of an unspecified Template:Mvar-dimensional vector space.

Motivation

By giving a collection of subspaces of some vector space a topological structure, it is possible to talk about a continuous choice of subspace or open and closed collections of subspaces; by giving them the structure of a differential manifold one can talk about smooth choices of subspace.

A natural example comes from tangent bundles of smooth manifolds embedded in Euclidean space. Suppose we have a manifold Template:Mvar of dimension Template:Mvar embedded in Rn. At each point Template:Mvar in Template:Mvar, the tangent space to Template:Mvar can be considered as a subspace of the tangent space of Rn, which is just Rr. The map assigning to Template:Mvar its tangent space defines a map from Template:Mvar to Gr(r, n). (In order to do this, we have to translate the geometrical tangent space to Template:Mvar so that it passes through the origin rather than Template:Mvar, and hence defines a Template:Mvar-dimensional vector subspace. This idea is very similar to the Gauss map for surfaces in a 3-dimensional space.)

This idea can with some effort be extended to all vector bundles over a manifold Template:Mvar, so that every vector bundle generates a continuous map from Template:Mvar to a suitably generalised Grassmannian—although various embedding theorems must be proved to show this. We then find that the properties of our vector bundles are related to the properties of the corresponding maps viewed as continuous maps. In particular we find that vector bundles inducing homotopic maps to the Grassmannian are isomorphic. But the definition of homotopic relies on a notion of continuity, and hence a topology.

Low dimensions

For r = 1, The Grassmannian Gr(1, 3) is the space of lines through the origin in 3-space, so it is the same as the projective plane.

For r = 2, the Grassmannian is the space of all planes through the origin. In Euclidean 3-space, a plane containing the origin is completely characterized by the one and only line through the origin perpendicular to that plane (and vice versa); hence Gr(2, 3) ≅ Gr(1, 3) ≅ P2, the projective plane.

The simplest Grassmannian that is not a projective space is Gr(2, 4), which may be parameterized via Plücker coordinates.

The Grassmannian as a set

Let Template:Mvar be a finite-dimensional vector space over a field Template:Mvar. The Grassmannian Gr(r, V) is the set of all Template:Mvar-dimensional linear subspaces of Template:Mvar. If Template:Mvar has dimension Template:Mvar, then the Grassmannian is also denoted Gr(r, n).

Vector subspaces of Template:Mvar are equivalent to linear subspaces of the projective space P(V), so it is equivalent to think of the Grassmannian as the set of all linear subspaces of P(V). When the Grassmannian is thought of this way, it is often written as Gr(r − 1, P(V)) or Gr(r − 1, n − 1).

The Grassmannian as a homogeneous space

The quickest way of giving the Grassmannian a geometric structure is to express it as a homogeneous space. First, recall that the general linear group GL(V) acts transitively on the Template:Mvar-dimensional subspaces of Template:Mvar. Therefore, if Template:Mvar is the set of stabilizers of this action, we have

Gr(r, V) = GL(V)/H.

If the underlying field is R or C and GL(V) is considered as a Lie group, then this construction makes the Grassmannian into a smooth manifold. It also becomes possible to use other groups to make this construction. To do this, fix an inner product on Template:Mvar. Over R, one replaces GL(V) by the orthogonal group O(V), and by restricting to orthonormal frames, one gets the identity

Gr(r, n) = O(n)/(O(r) × O(nr)).

In particular, the dimension of the Grassmannian is r(nr).

Over C, one replaces GL(V) by the unitary group U(V). This shows that the Grassmannian is compact. These constructions also make the Grassmannian into a metric space: For a subspace Template:Mvar of Template:Mvar, let PW be the projection of Template:Mvar onto Template:Mvar. Then

where Template:!!Template:!! denotes the operator norm, is a metric on Gr(r, V). The exact inner product used does not matter, because a different inner product will give an equivalent norm on Template:Mvar, and so give an equivalent metric.

If the ground field Template:Mvar is arbitrary and GL(V) is considered as an algebraic group, then this construction shows that the Grassmannian is a non-singular algebraic variety. It follows from the existence of the Plücker embedding that the Grassmannian is complete as an algebraic variety. In particular, Template:Mvar is a parabolic subgroup of GL(V).

The Grassmannian as a scheme

In the realm of algebraic geometry, the Grassmannian can be constructed as a scheme by expressing it as a representable functor.[2]

Representable functor

Let be a quasi-coherent sheaf on a scheme Template:Mvar. Fix a positive integer Template:Mvar. Then to each Template:Mvar-scheme Template:Mvar, the Grassmannian functor associates the set of quotient modules of

locally free of rank Template:Mvar on Template:Mvar. We denote this set by .

This functor is representable by a separated Template:Mvar-scheme . The latter is projective if is finitely generated. When Template:Mvar is the spectrum of a field Template:Mvar, then the sheaf is given by a vector space Template:Mvar and we recover the usual Grassmannian variety of the dual space of Template:Mvar, namely: Gr(r, V).

By construction, the Grassmannian scheme is compatible with base changes: for any Template:Mvar-scheme S′, we have a canonical isomorphism

In particular, for any point Template:Mvar of Template:Mvar, the canonical morphism {s} = Spec(k(s)) → S, induces an isomorphism from the fiber to the usual Grassmannian over the residue field k(s).

Universal family

Since the Grassmannian scheme represents a functor, it comes with a universal object, , which is an object of

and therefore a quotient module of , locally free of rank Template:Mvar over . The quotient homomorphism induces a closed immersion from the projective bundle :

For any morphism of Template:Mvar-schemes:

this closed immersion induces a closed immersion

Conversely, any such closed immersion comes from a surjective homomorphism of OT-modules from to a locally free module of rank Template:Mvar.[3] Therefore, the elements of are exactly the projective subbundles of rank Template:Mvar in

Under this identification, when T = S is the spectrum of a field Template:Mvar and is given by a vector space Template:Mvar, the set of rational points correspond to the projective linear subspaces of dimension r − 1 in P(V), and the image of in

is the set

The Plücker embedding

{{#invoke:main|main}} The Plücker embedding is a natural embedding of a Grassmannian into a projective space:

Suppose that Template:Mvar is an Template:Mvar-dimensional subspace of Template:Mvar. To define ψ(W), choose a basis {w1, ..., wr}, of Template:Mvar, and let ψ(W) be the wedge product of these basis elements:

A different basis for Template:Mvar will give a different wedge product, but the two products will differ only by a non-zero scalar (the determinant of the change of basis matrix). Since the right-hand side takes values in a projective space, Template:Mvar is well-defined. To see that Template:Mvar is an embedding, notice that it is possible to recover Template:Mvar from ψ(W) as the set of all vectors Template:Mvar such that wψ(W) = 0.

The embedding of the Grassmannian satisfies some very simple quadratic polynomials called the Plücker relations. These show that the Grassmannian embeds as an algebraic subvariety of P(∧rV) and give another method of constructing the Grassmannian. To state the Plücker relations, choose two Template:Mvar-dimensional subspaces Template:Mvar and Template:Mvar of Template:Mvar with bases {w1, ..., wr}, and {z1, ..., zr}, respectively. Then, for any integer k ≥ 0, the following equation is true in the homogeneous coordinate ring of P(∧rV):

When dim(V) = 4, and r = 2, the simplest Grassmannian which is not a projective space, the above reduces to a single equation. Denoting the coordinates of P(∧rV) by X1,2, X1,3, X1,4, X2,3, X2,4, X3,4, we have that Gr(2, V) is defined by the equation

X1,2X3,4X1,3X2,4X2,3X1,4 = 0.

In general, however, many more equations are needed to define the Plücker embedding of a Grassmannian in projective space.

The Grassmannian as a real affine algebraic variety

Let Gr(r, Rn) denote the Grassmannian of Template:Mvar-dimensional subspaces of Rn. Let M(n, R) denote the space of real n × n matrices. Consider the set of matrices A(r, n) ⊂ M(n, R) defined by XA(r, n) if and only if the three conditions are satisfied:

A(r, n) and Gr(r, Rn) are homeomorphic, with a correspondence established by sending XA(r, n) to the column space of Template:Mvar.

Duality

Every Template:Mvar-dimensional subspace Template:Mvar of Template:Mvar determines an (nr)-dimensional quotient space V/W of Template:Mvar. This gives the natural short exact sequence:

0 → WVV/W → 0.

Taking the dual to each of these three spaces and linear transformations yields an inclusion of (V/W) in V with quotient W:

0 → (V/W)VW → 0.

Using the natural isomorphism of a finite-dimensional vector space with its double dual shows that taking the dual again recovers the original short exact sequence. Consequently there is a one-to-one correspondence between Template:Mvar-dimensional subspaces of Template:Mvar and (nr)-dimensional subspaces of V. In terms of the Grassmannian, this is a canonical isomorphism

Gr(r, V) ≅ Gr(nr, V).

Choosing an isomorphism of Template:Mvar with V therefore determines a (non-canonical) isomorphism of Gr(r, V) and Gr(nr, V). An isomorphism of Template:Mvar with V is equivalent to a choice of an inner product, and with respect to the chosen inner product, this isomorphism of Grassmannians sends an Template:Mvar-dimensional subspace into its (nr)-dimensional orthogonal complement.

Schubert cells

The detailed study of the Grassmannians uses a decomposition into subsets called Schubert cells, which were first applied in enumerative geometry. The Schubert cells for Gr(r, n) are defined in terms of an auxiliary flag: take subspaces V1, V2, ..., Vr, with ViVi + 1. Then we consider the corresponding subset of Gr(r, n), consisting of the Template:Mvar having intersection with Vi of dimension at least Template:Mvar, for i = 1, ..., r. The manipulation of Schubert cells is Schubert calculus.

Here is an example of the technique. Consider the problem of determining the Euler characteristic of the Grassmannian of Template:Mvar-dimensional subspaces of Rn. Fix a 1-dimensional subspace RRn and consider the partition of Gr(r, n) into those Template:Mvar-dimensional subspaces of Rn that contain R and those that do not. The former is Gr(r − 1, n − 1) and the latter is a Template:Mvar-dimensional vector bundle over Gr(r, n − 1). This gives recursive formulas:

If one solves this recurrence relation, one gets the formula: χr, n = 0 if and only if Template:Mvar is even and Template:Mvar is odd. Otherwise:

Cohomology ring of the complex Grassmannian

Every point in the complex Grassmannian manifold Gr(r, n) defines an Template:Mvar-plane in Template:Mvar-space. Fibering these planes over the Grassmannian one arrives at the vector bundle Template:Mvar which generalizes the tautological bundle of a projective space. Similarly the (nr)-dimensional orthogonal complements of these planes yield an orthogonal vector bundle Template:Mvar. The integral cohomology of the Grassmannians is generated, as a ring, by the Chern classes of Template:Mvar. In particular, all of the integral cohomology is at even degree as in the case of a projective space.

These generators are subject to a set of relations, which defines the ring. The defining relations are easy to express for a larger set of generators, which consists of the Chern classes of Template:Mvar and Template:Mvar. Then the relations merely state that the direct sum of the bundles Template:Mvar and Template:Mvar is trivial. Functoriality of the total Chern classes allows one to write this relation as

The quantum cohomology ring was calculated by Edward Witten in The Verlinde Algebra And The Cohomology Of The Grassmannian. The generators are identical to those of the classical cohomology ring, but the top relation is changed to

reflecting the existence in the corresponding quantum field theory of an instanton with 2n fermionic zero-modes which violates the degree of the cohomology corresponding to a state by 2n units.

Associated measure

When Template:Mvar is Template:Mvar-dimensional Euclidean space, one may define a uniform measure on Gr(r, n) in the following way. Let θn be the unit Haar measure on the orthogonal group O(n) and fix Template:Mvar in Gr(r, n). Then for a set AGr(r, n), define

This measure is invariant under actions from the group O(n), that is, γr, n(gA) = γr, n(A) for all Template:Mvar in O(n). Since θn(O(n)) = 1, we have γr, n(Gr(r, n)) = 1. Moreover, γr, n is a Radon measure with respect to the metric space topology and is uniform in the sense that every ball of the same radius (with respect to this metric) is of the same measure.

Oriented Grassmannian

This is the manifold consisting of all oriented Template:Mvar-dimensional subspaces of Rn. It is a double cover of Gr(r, n) and is denoted by:

As a homogeneous space can be expressed as:

Applications

Grassmann manifolds have found application in computer vision tasks of video-based face recognition and shape recognition.[4]

Grassmannians allow the scattering amplitudes of subatomic particles to be calculated via a positive Grassmannian construct called the amplituhedron.[5]

See also

Notes

  1. Template:Harvtxt, pp. 57–59.
  2. {{#invoke:citation/CS1|citation |CitationClass=book }}, Chapter I.9
  3. EGA, II.3.6.3.
  4. Pavan Taruga, Ashok Veeraraghavan, Rama Chellappa: Statistical analysis on Stiefel and Grassmann manifolds with applications in computer vision, CVPR 23–28 June 2008, IEEE Conference on Computer Vision and Pattern Recognition, 2008, ISBN 978-1-4244-2242-5, pp. 1–8 (abstract, full text)
  5. {{#invoke:Citation/CS1|citation |CitationClass=journal }}
  6. {{#invoke:Citation/CS1|citation |CitationClass=journal }}, see section 4.3., pp. 137–140

References

  • {{#invoke:Citation/CS1|citation

|CitationClass=journal }} section 1.2

  • {{#invoke:citation/CS1|citation

|CitationClass=book }} see chapters 5-7

  • Joe Harris, Algebraic Geometry, A First Course, (1992) Springer, New York, ISBN 0-387-97716-3
  • Pertti Mattila, Geometry of Sets and Measures in Euclidean Spaces, (1995) Cambridge University Press, New York, ISBN 0-521-65595-1