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In [[mathematics]], the '''Lagrangian Grassmannian''' is the [[smooth manifold]] of [[Lagrangian subspace]]s of a real [[symplectic vector space]] ''V''. Its dimension is ''n(n+1)/2'' (where the dimension of ''V'' is ''2n''). It may be identified with the [[homogeneous space]] | |||
:''U''(''n'')/''O''(''n''), | |||
where ''U''(''n'') is the [[unitary group]] and ''O''(''n'') the [[orthogonal group]]. Following [[Vladimir Arnold]] it is denoted by Λ(''n''). The Lagrangian Grassmannian is a submanifold of the ordinary [[Grassmannian]] of '''V'''. | |||
A ''' complex Lagrangian Grassmannian''' is the [[homogeneous space|complex homogeneous manifold]] of [[Lagrangian subspace]]s of a complex [[symplectic vector space]] ''V'' of dimension 2''n''. It may be identified with the [[homogeneous space]] of complex dimension ''n(n+1)/2'' | |||
:''Sp''(''n'')/''U''(''n''), | |||
where ''Sp''(''n'') is the [[symplectic group|compact symplectic group]]. | |||
==Topology== | |||
The stable topology of the Lagrangian Grassmannian and complex Lagrangian Grassmannian is completely understood, as these spaces appear in the [[Bott periodicity theorem]]: <math>\Omega(Sp/U) \simeq U/O</math>, and <math>\Omega(U/O) \simeq Z\times BO</math> – they are thus exactly the [[Orthogonal_group#Homotopy_groups|homotopy groups of the stable orthogonal group]], up to a shift in indexing (dimension). | |||
In particular, the [[fundamental group]] of <math>U/O</math> is [[infinite cyclic]], with a distinguished generator given by the square of the [[determinant]] of a [[unitary matrix]], as a mapping to the [[unit circle]]. Its first [[homology group]] is therefore also infinite cyclic, as is its first cohomology group. Arnold showed that this leads to a description of the '''Maslov index''', introduced by [[V. P. Maslov]]. | |||
For a [[Lagrangian submanifold]] ''M'' of ''V'', in fact, there is a mapping | |||
:''M'' → Λ(''n'') | |||
which classifies its [[tangent space]] at each point (cf. [[Gauss map]]). The Maslov index is the pullback via this mapping, in | |||
:''H''<sup>1</sup>(''M'', '''Z''') | |||
of the distinguished generator of | |||
:''H''<sup>1</sup>(Λ(''n''), '''Z'''). | |||
==Maslov index== | |||
A path of [[symplectomorphism]]s of a symplectic vector space may be assigned a '''Maslov index''', named after [[V. P. Maslov]]; it will be an integer if the path is a loop, and a half-integer in general. | |||
If this path arises from trivializing the [[symplectic vector bundle]] over a periodic orbit of a [[Hamiltonian vector field]] on a [[symplectic manifold]] or the [[Reeb vector field]] on a [[contact manifold]], it is known as the [[Conley-Zehnder index]]. It computes the [[spectral flow]] of the [[Cauchy-Riemann equations|Cauchy-Riemann]]-type operators that arise in [[Floer homology]]{{Citation needed|date=October 2013}}. | |||
It appeared originally in the study of the [[WKB approximation]] and appears frequently in the study of [[Bohr-Sommerfeld_quantization|quantization]] and in [[symplectic geometry]] and topology. It can be described as above in terms of a Maslov index for linear Lagrangian submanifolds. | |||
==References== | |||
*V. I. Arnold, '' Characteristic class entering in quantization conditions'', Funktsional'nyi Analiz i Ego Prilozheniya, '''1967''', 1,1, 1-14, {{doi|10.1007/BF01075861}}. | |||
*[[V. P. Maslov]], ''Théorie des perturbations et méthodes asymptotiques''. 1972 | |||
*{{citation|url=http://www.maths.ed.ac.uk/~aar/maslov.htm |title=The Maslov index home page |first=Andrew|last= Ranicki}} Assorted source material relating to the Maslov index. | |||
[[Category:Symplectic geometry]] | |||
[[Category:Topology of homogeneous spaces]] | |||
[[Category:Mathematical quantization]] |
Revision as of 17:31, 21 March 2013
In mathematics, the Lagrangian Grassmannian is the smooth manifold of Lagrangian subspaces of a real symplectic vector space V. Its dimension is n(n+1)/2 (where the dimension of V is 2n). It may be identified with the homogeneous space
- U(n)/O(n),
where U(n) is the unitary group and O(n) the orthogonal group. Following Vladimir Arnold it is denoted by Λ(n). The Lagrangian Grassmannian is a submanifold of the ordinary Grassmannian of V.
A complex Lagrangian Grassmannian is the complex homogeneous manifold of Lagrangian subspaces of a complex symplectic vector space V of dimension 2n. It may be identified with the homogeneous space of complex dimension n(n+1)/2
- Sp(n)/U(n),
where Sp(n) is the compact symplectic group.
Topology
The stable topology of the Lagrangian Grassmannian and complex Lagrangian Grassmannian is completely understood, as these spaces appear in the Bott periodicity theorem: , and – they are thus exactly the homotopy groups of the stable orthogonal group, up to a shift in indexing (dimension).
In particular, the fundamental group of is infinite cyclic, with a distinguished generator given by the square of the determinant of a unitary matrix, as a mapping to the unit circle. Its first homology group is therefore also infinite cyclic, as is its first cohomology group. Arnold showed that this leads to a description of the Maslov index, introduced by V. P. Maslov.
For a Lagrangian submanifold M of V, in fact, there is a mapping
- M → Λ(n)
which classifies its tangent space at each point (cf. Gauss map). The Maslov index is the pullback via this mapping, in
- H1(M, Z)
of the distinguished generator of
- H1(Λ(n), Z).
Maslov index
A path of symplectomorphisms of a symplectic vector space may be assigned a Maslov index, named after V. P. Maslov; it will be an integer if the path is a loop, and a half-integer in general.
If this path arises from trivializing the symplectic vector bundle over a periodic orbit of a Hamiltonian vector field on a symplectic manifold or the Reeb vector field on a contact manifold, it is known as the Conley-Zehnder index. It computes the spectral flow of the Cauchy-Riemann-type operators that arise in Floer homologyPotter or Ceramic Artist Truman Bedell from Rexton, has interests which include ceramics, best property developers in singapore developers in singapore and scrabble. Was especially enthused after visiting Alejandro de Humboldt National Park..
It appeared originally in the study of the WKB approximation and appears frequently in the study of quantization and in symplectic geometry and topology. It can be described as above in terms of a Maslov index for linear Lagrangian submanifolds.
References
- V. I. Arnold, Characteristic class entering in quantization conditions, Funktsional'nyi Analiz i Ego Prilozheniya, 1967, 1,1, 1-14, 21 year-old Glazier James Grippo from Edam, enjoys hang gliding, industrial property developers in singapore developers in singapore and camping. Finds the entire world an motivating place we have spent 4 months at Alejandro de Humboldt National Park..
- V. P. Maslov, Théorie des perturbations et méthodes asymptotiques. 1972
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