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[parent] symplectic complement (Definition)

Definition [1,2] Let $ (V,\omega)$ be a symplectic vector space and let $ W$ be a vector subspace of $ V$. Then the symplectic complement of $ W$ is

$\displaystyle W^\omega = \{x\in V\, \vert \, \omega(x,y)=0\,\,$   for all$\displaystyle \,\, y\in W\}.$

It is easy to see that $ W^\omega$ is also a vector subspace of $ V$. Depending on the relation between $ W$ and $ W^\omega$, $ W$ is given different names.

  1. If $ W\subset W^\omega$, then $ W$ is an isotropic subspace (of $ V$).
  2. If $ W^\omega \subset W$, then $ W$ is an coisotropic subspace.
  3. If $ W \cap W^\omega=\{0\}$, then $ W$ is an symplectic subspace.
  4. If $ W = W^\omega$, then $ W$ is an Lagrangian subspace.

For the symplectic complement, we have the following dimension theorem.

Theorem [1,2] Let $ (V,\omega)$ be a symplectic vector space, and let $ W$ be a vector subspace of $ V$. Then

$\displaystyle \dim V = \dim W^\omega + \dim W.$

Bibliography

1
D. McDuff, D. Salamon, Introduction to Symplectic Topology, Clarendon Press, 1997.
2
R. Abraham, J.E. Marsden, Foundations of Mechanics, 2nd ed., Perseus Books, 1978.



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Also defines:  symplectic complement, isotropic subspace, coisotropic subspace, symplectic subspace, Lagrangian subspace

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dimension theorem for symplectic complement (proof) (Proof) by matte
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Cross-references: dimension, relation, easy to see, vector subspace, symplectic vector space
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This is version 5 of symplectic complement, born on 2003-04-02, modified 2004-02-28.
Object id is 4139, canonical name is SymplecticComplement.
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Classification:
AMS MSC15A04 (Linear and multilinear algebra; matrix theory :: Linear transformations, semilinear transformations)

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