SYMPLECTIC DECOMPOSITION OF SYMPLECTIC SUBSPACES
SYMPLECTIC DECOMPOSITION OF SYMPLECTIC SUBSPACES
The Pure and Applied Mathematics. 2015. Feb, 22(1): 13-23
• Received : July 27, 2014
• Accepted : October 08, 2014
• Published : February 28, 2015 PDF e-PUB PubReader PPT Export by style
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JAE-HYOUK, LEE

Abstract
We introduce a decomposition on a symplectic subspace determined by symplectic structure and study its properties. As a consequence, we give an elementary proof of the deformation of the Grassmannians of symplectic subspaces to the complex Grassmannians.
Keywords
1. INTRODUCTION
On a 2 n -dimensional symplectic vector space V with a symplectic structure 𝜔, the set of all 2 k -dimensional symplectic subspaces S V is called the symplectic Grassmannian Gr Sp (2 k , 2 n ). This is an open subset of the real Grassmannian Gr (2 k , 2 n ) and contains the complex Grassmannian PPT Slide
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consisted of those subspaces S which are complex for a linear complex structure on V .
In  , the symplectic Grassmannian Gr Sp (2 k , 2 n ) is introduced and identified as PPT Slide
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and we discussed natural inclusions PPT Slide
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The inclusion Gr Sp (2 k , 2 n ) ⊂ Gr (2 k , 2 n ) is the complement of a hyperplane in 𝜦 2k V , and the inclusion PPT Slide
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is the intersection with PPT Slide
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, and topologically it is a deformation retract. In particular, the deformation retract of Gr Sp (2 k , 2 n ) is as natural as the deformation from PPT Slide
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to U ( n ) which is related to the Siegel upper half space PPT Slide
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representing the space of all compatible complex structures on ( V , 𝜔). But the proof in  contains tedious issues left to the reader, and it turns out that many readers find diffculties to fill in the detail of arguments. The objective of this paper is to show the deformation retract of Gr Sp (2 k , 2 n ) via rather elementary methods.
We introduce a notion of 𝜔- basis and 𝜔- decomposition of a symplectic subspace S in ( V , 𝜔) determined by the symplectic structure 𝜔 on V (Definition 4), and show that the decomposition is uniquely determined (Lemma 3). Moreover, we obtain the following key Theorem 7,
Theorem. For each 2 k - dimensional symplectic subspace S in a 2 n - dimensional Hermitian vector space ( V , 𝜔, J , g ), there is an 1- parameter family of symplectic subspaces S ( t ), t ∊ [0, 1] in V such that S (0) = S and S (1) is a complex subspace in V . Moreover , it is uniquely determined by the 𝜔- decomposition of S .
As a consequence we show that complex Grassmannian PPT Slide
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is a strong deformation retract of the symplectic Grassmannian Gr Sp (2 k , 2 n ) (Theorem 9)
In  , each element in Gr Sp (2 k , 2 n ) is considered as an element of 𝜦 2k V but in this paper we rather consider it as a linear subspace.
2. PRELIMINARIES
In this section, we recall the definition of symplectic Grassmannians and discuss their basic properties and their relationships with real and complex Grassmannians in  .
In this article, V always denotes a 2 n -dimensional symplectic vector space with symplectic form 𝜔. Moreover, we consider a Hermitian structure (𝜔, J , g ) on V where J is a complex structure and g is a metric structure on V satisfying
• 𝜔 (u,v) =g(Ju,v) ,g(Ju,Jv) =g(u,v)
for any u , v V . Note that any two among symplectic, complex and metric structures on V are called compatible with each other if they define the third structure via the relation 𝜔 ( u , v ) = g ( Ju , v ) and together they make V into a Hermitian vector space.
Symplectic complement, null space and symplectic rank For any linear subspace P in symplectic vector space ( V , 𝜔), we recall the following notions.
• (1) Thesymplectic complementofPisP𝜔:= {u∊V｜ 𝜔 (u,v) = 0 for allv∊P}
• (2) ThenullspaceofPisN(P) :=P∩P𝜔and its dimension is called the nullityn(P) := dimN(P)
• (3) Thesymplectic rank,or simply rank, ofPisr(P) :=
• (4) Any two vectorsuandvwith 𝜔 (u,v) = 0 (resp.g(u,v) = 0) are called 𝜔-orthogonal(resp.g-orthogonal)
The null space N ( P ) is the largest subset of P where the restriction of 𝜔 vanishes, and the rank r ( P ) is half of the maximal possible dimension among subspaces in P where 𝜔 is nondegenerate.
Symplectic complements satisfy the following basic properties:
• (i) (P𝜔)𝜔=P, (ii) (P∩Q)𝜔=P𝜔+Q𝜔and (iii)P⊂QiffP𝜔⊃Q𝜔.
And we also have dim P = 2 · r ( P ) + n ( P ).
Subspaces in symplectic spaces A linear subspace S of V is called symplectic if the restriction of 𝜔 to S defines a symplectic structure on S , equivalently n ( S ) = 0. Thus, S must be of even dimension. And a subspace P is called isotropic (resp. coisotropic ) if P P 𝜔 (resp. P 𝜔 P ), and this condition is equivalent to 𝜔｜ P = 0; namely P = N ( P ) (resp. P 𝜔 = N ( P )). When a subspace is both isotropic and coisotropic, we call it Lagrangian .
In particular, one can obtain the following equivalent statements for symplectic subspaces : (1) P is a symplectic subspace in V ; (2) n ( P ) = 0; (3) dim P = 2 · r ( P ) and (4) V = P P 𝜔 .
It is well-known that the symplectic complement of a symplectic (resp. isotropic) subspace in ( V , 𝜔) is symplectic (resp. coisotropic). As a matter of fact, the similar statements for orthogonal complements are also true (See  for detail).
Lemma 1 . Let P be any linear subspace in a Hermitian vector space ( V , 𝜔, J , g ). For the orthogonal complement P of P in V , we have the following .
• (1)JN(P) =N(P⊥)
• (2)P is symplectic iffP⊥is symplectic.
• (3)P is coisotropic iffP⊥is isotropic.
Symplectic Grassmannians
Definition 2. Given any k n , the set of all 2 k -dimensional symplectic linear subspaces in a 2 n -dimensional symplectic vector space V is called the symplectic Grassmannian , and it is denoted as Gr Sp (2 k , V ), or simply Gr Sp (2 k , 2 n ).
Because the symplectic condition on a subspace is an open condition, a natural choice of topology for the symplectic Grassmannian is one given by being an open subset of the real Grassmannian Gr (2 k , 2 n ) of all 2 k -dimensional real linear subspaces in V .
We pick a metric g on V so that we obtain orthogonal decomposition V = P P for each subspace P in V . By considering the orthonormal bases of P and P which also give one on V , we identify Gr (2 k , 2 n ) with a homogeneous space of O (2 n ) with the isotropy subgroup the product of orthogonal groups of P and P . Therefore, we have Gr (2 k , 2 n ) ≃ O (2 n ) / O (2 k ) O (2 n – 2 k ). Moreover, for the same reason, there is also a canonical identification between Gr Sp (2 k , 2 n ) and the homogeneous space PPT Slide
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and if we equip a complex vector space ( V , J ) with a Hermitian metric g , then the complex Grassmannian PPT Slide
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can be identified with U ( n ) / U ( k ) U ( n k ).
Therefrom we have the canonical inclusions PPT Slide
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correspond to the following inclusions of homogeneous spaces PPT Slide
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3. RETRACT OF SYMPLECTIC GRASSMANNIANS
By Lemma 1, the symplectic Grassmannians Gr Sp (2 k , 2 n ) and Gr Sp (2 n – 2 k , 2 n ) are dual to each other. Thus we could restrict our attention to only those subspaces in V which are at most half dimensional. For the rest of this section, we assume that 2 k n .
In  , PPT Slide
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acts on PPT Slide
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, it also acts on Gr (2 k , 2 n ) which is the space of linear subspaces of V . we have a disjoint union decomposition PPT Slide
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where 𝓞 r is the orbit of rank k subspaces. 𝓞 k = Gr Sp (2 k , 2 n ) is the unique open orbit in Gr (2 k , 2 n ). In particular the complement of Gr Sp (2 k , 2 n ) is a hypersurface in Gr (2 k , 2 n ).
On the other hands, we recall PPT Slide
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is a calibration satisfying the Wirtinger’s inequality which is PPT Slide
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and equality sign holds if and only if 𝜍 is complex subspace, i.e. PPT Slide
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(see  ). Here 𝜔 k (𝜍) denotes
• 𝜔k(e1∧e2... ∧e2k)
where e 1 , e 2 , ..., e 2k is an oriented orthonormal basis of 𝜍. Equivalently, PPT Slide
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is a calibration with contact set PPT Slide
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. Note we have PPT Slide
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In the below, we show that PPT Slide
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is actually a strong deformation retract of Gr Sp (2 k , 2 n ).
𝜔- basis and 𝜔- decomposition of symplectic subspaces
Lemma 3. Let ( V , 𝜔, J , g ) be a 2 n - dimensional Hermitian vector space . For any 2 k - dimensional symplectic subspace S in V , there is an ordered orthonormal basis { u 1 , v 1 , ..., u k , v k } on S such that
• 𝜔 (u1,v1) ≥ 𝜔 (u2,v2) ≥ ... ≥ 𝜔 (uk,vk) > 0
and 𝜔- orthogonal for other pairs in the basis .
Proof . For each oriented pair of orthonormal vectors u and v in S , we consider
• 𝜔 (u,v) =g(Ju,v) .
And there is a maximizing oriented orthonormal pair of vectors u 1 and v 1 in S satisfying PPT Slide
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. Here, g ( J u 1 , v 1 ) is nonzero because S is symplectic.
For each unit vector w in S span { u 1 , v 1 } , we observe that the function
• f(𝜽) :=g(Ju1,cos𝜽v1+ sin 𝜽w)
has a maximum value at 𝜽 = 0 and hence the derivative of f at 𝜽 = 0 vanishes, 0 = f ′ (0) = g ( Ju 1 , w ), i.e. w Ju 1 . Similarly, we also obtain w Jv 1 . Therefore any g -orthogonal vector w to span { u 1 , v 1 } is also 𝜔-orthogonal, and a maximizing pair ( u 1 , v 1 ) is isolated.
By Lemma 1, S span { u 1 , v 1 } is symplectic. Thus we can repeat the above process for S span { u 1 , v 1 } and obtain an ordered orthonormal basis { u 1 , v 1 , ..., u k , v k } of S such that
• 𝜔 (u1,v1) =g(Ju1,v1) ≥g(Ju2,v2) ≥ ... ≥g(Juk,vk) > 0,
and each pair of vectors in the basis is 𝜔-orthogonal except ( u 1 , v 1 ) i = 1, ..., k . PPT Slide
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Because the value of g ( Ju i , v i ) can be repeated, we give a refined index to the basis in the above Lemma 3 so as to have the following definition.
Definition 4. Let ( V , 𝜔, J , g ) be a 2 n -dimensional Hermitian vector space. For any 2 k -dimensional symplectic subspace S in V , an ordered orthonormal basis PPT Slide
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on S such that
• 1 ≥ λ1 > λ2 > ... > λm > 0
where PPT Slide
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, i = 1, ..., m , and 𝜔-orthogonal for other pairs in the basis is called a 𝜔- basis . Moreover, we call each PPT Slide
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in S a λ i -component of S and S = S λ1 ⊕ ... ⊕ S λm a 𝜔-decomposition (or symplectic decomposition) of S .
Remark. The referee comments that the 𝜔-decomposition is simply the decomposition of the eigenspaces of A 2 , where A is the non-degenerate skew-symmetric matrix associated to the symplectic form 𝜔.
It is natural to ask the uniqueness of 𝜔-decomposition of a symplectic subspace S in ( V , 𝜔, J , g ). At first, we consider the following Lemma.
Lemma 5. Let S be a 2 k - dimensional symplectic subspace in a 2 n - dimensional Hermitian vector space ( V , 𝜔, J , g ) and let PPT Slide
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be a 𝜔- basis on S as in Definition 4 . If u , v are ordered orthonormal vectors in S with 𝜔 ( u , v ) = λ 1 , then span { u , v } ⊂ S λ1 .
Proof . By applying the 𝜔-decomposition of S given by the basis B , u can be written as u = a 0 u 0 + a + u + where u 0 and v + are orthonormal vectors in S λ1 and PPT Slide
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, respectively. Since u is a unit vector, the coeffcients a 0 and a + satisfy PPT Slide
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. Similarly, v can be written as v = b 0 v 0 + b + v + , and we have PPT Slide
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.
Since S λ1 and PPT Slide
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are also 𝜔-orthogonal, we get 𝜔 ( u 0 , v + ) = 𝜔 ( v 0 , u + ) = 0, and we obtain PPT Slide
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Here we use the fact that the value of 𝜔 on each S 𝜆i is in [-𝜆 i , 𝜆 i ] so that we have 𝜔 2 ( u 0 , v 0 ) ≤ 𝜆 1 and 𝜔 2 ( u + , v + ) ≤ 𝜆 2 . To get the equality in the above, we need a + = b + = 0 so that u and v are in S 𝜆1 . This gives the Lemma 5. PPT Slide
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By applying Lemma 5 inductively, we conclude the following Theorem.
Theorem 6. For each 2 k - dimensional symplectic subspace S in a 2 n - dimensional Hermitian vector space ( V , 𝜔, J , g ), the 𝜔- decomposition of S is uniquely determined .
Now, for each symplectic subspace S , we define an 1-paramameter family of symplectic subspaces which in fact gives a path from [ S ] in Gr Sp (2 k , 2 n ) to an element in PPT Slide
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.
Theorem 7. For each 2 k - dimensional symplectic subspace S in a 2 n - dimensional Hermitian vector space ( V , 𝜔, J , g ), there is an 1- parameter family of symplectic subspaces S ( t ), t ∊ [0, 1] in V such that S (0) = S and S (1) is a complex subspace in V . Moreover, it is uniquely determined by the 𝜔- decomposition of S .
Proof . By Lemma 3, we obtain a 𝜔-basis { u 1 , v 1 , ..., u k , v k } of S such that
• g(Ju1,v1) ≥g(Ju2,v2) ≥ ... ≥g(Juk,vk) > 0,
and S = span { u 1 , v 1 } ⊕... ⊕ span { u k , v k }. And for each 0 ≤ t ≤ 1 and i = 1, ..., k , we define PPT Slide
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and consider
• S(t) :=span{U1(t) ,V1(t) , ...,Uk(t) ,Vk(t)} .
One can check that S ( t) is symplectic for each t , and S (0) = S and S (1) is a complex subspace in V .
To show the family is uniquely determined by the 𝜔-decomposition of S , we need to show that the construction of symplectic subspace S ( t ) is independent of the choice of the 𝜔-basis on S .
First, we observe { U 1 ( t ) , V 1 ( t ) , ..., U k ( t ) , V k ( t )} is a 𝜔-basis of S ( t ).
By applying PPT Slide
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one can check that
• (i) {U1(t) ,V1(t) , ...,Uk(t) ,Vk(t)} is an orthonormal basis ofS(t),
• (ii) 𝜔 (U1(t) ,V1(t)) ≥ 𝜔 (U2(t) ,V2(t)) ≥ ... ≥ 𝜔 (Uk(t) ,Vk(t)) > 0, and
• (iii) 𝜔-orthogonal for other pairs in {U1(t) ,V1(t) , ...,Uk(t) ,Vk(t)}.
Thus { U 1 ( t ) , V 1 ( t ) , ..., U k ( t ) , V k ( t )} is a 𝜔-basis of S ( t ), indeed.
It is useful to note that if 𝜔 ( u i , v i ) = 𝜔 ( u j , v j ), then 𝜔 ( U i ( t ) , V i ( t )) = 𝜔 ( U j ( t ) , V j ( t )). Therefore if span { u i , v i } and span { u j , v j } are in the same component of 𝜔-decomposition of S , span { U i ( t ) , V i ( t )} and span { U j ( t ) , V j ( t )} must be in the same component of 𝜔-decomposition of S ( t ) for each t .
Second, we want to show that all 𝜔-basis on S produces the same S ( t ) at each t having the same 𝜔-decomposition of S ( t ).
Suppose { u 1 , v 1 , ..., u m , v m } in the given basis { u 1 , v 1 , ..., u k , v k } forms the λ 1- component of S , i.e.
• λ1= 𝜔 (u1,v1) = ... = 𝜔 (um,vm)
and
• Sλ1=span{u1,v1, ...,um,vm} .
For another 𝜔-basis B' on S , by Lemma 5 B' has an ordered orthonormal pair u , v contained in S λ1 with 𝜔 ( u , v ) = λ 1 . As the above, we define U ( t ) and V ( t ) PPT Slide
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and we want to show that U ( t ) and V ( t ) are in span { U 1 ( t ) , V 1 ( t ) , ..., U m ( t ) , V m ( t )} for each t .
Since u and v are in S λ1 , we write u and v as
• u=x1u1+y1v1+ ... +xmum+ymvm
• v=z1u1+w1v1+ ... +zmum+wmvm,
where the coeffcients are real numbers. Since u and v are unit vectors, we have PPT Slide
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and because 𝜔 ( u , v ) = λ 1 , we get PPT Slide
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Here we use the fact that { u 1 , v 1 , ..., u m , v m } is a subset of the 𝜔-basis producing λ 1 -component in S . By combining above three equations in ( A -2) and ( A -3), we obtain PPT Slide
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which implies x i = w i and y i = – z i for each i , and that u and v can be written PPT Slide
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Now, U ( t ) and V ( t ) can be written PPT Slide
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which shows that U ( t ) and V ( t ) are in span { U 1 ( t ) , V 1 ( t ) , ..., U m ( t ) , V m ( t )}. Here we use
• g(v+Ju,v+Ju) =g(–u+Jv,–u+Jv) = 2 + 2𝜔 (u,v) = 2 + 2𝜔 (ui,vi) =g(vi+Jui,vi+Jui) =g(–ui+Jvi,–ui+Jvi)
for i = 1, ..., m .
By applying this procedure inductively, we can conclude that all 𝜔-basis on S produces the same S ( t ) at each t having the same 𝜔-decomposition of S ( t ). PPT Slide
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From the proof of the Theorem 7, we observe that the expression of u and v in ( A -2) is related to PPT Slide
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Therefore, the set of all the 𝜔-bases on a 2 k -dimensional symplectic subspace S is acted by block diagonal unitary matrices in U ( k ) ⊂ O (2 k ). Thus we have the following corollary.
Corollary 8. Let S be a 2 k - dimensional symplectic subspace in a 2 n - dimensional Hermitian vector space ( V , 𝜔, J , g ). Suppose S has a 𝜔- basis as in Definition 4 , then the set of all the 𝜔- bases on S is acted by U ( a 1 , a 2 , ..., a m ) which is the set of block diagonal unitary matrices in PPT Slide
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whose blocks have sizes (2 a 1 , 2 a 2 , ..., 2 a m ) as real matrices .
Remark. From ( A -4) and ( A -5) in Theorem 7, we conclude that the action of block diagonal unitary matrices U ( a 1 , a 2 , ..., a m ) on the set of all the 𝜔-bases on S and the set of all the 𝜔-bases on S ( t ) is compatible via the definition of vectors at t in ( A -1).
Now we show that PPT Slide
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is a strong deformation retract of Gr Sp (2 k , 2 n ), indeed.
Theorem 9. Let ( V , 𝜔, J , g ) be a 2 n - dimensional Hermitian vector space. Then the complex Grassmannian PPT Slide
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is a strong deformation retract of the symplectic Grassmannian Gr Sp (2 k , 2 n ).
Proof . First, we take a 𝜔-basis on S for each [ S ] in Gr Sp (2 k , 2 n ). By applying Theorem 7 for S and the 𝜔-basis, we obtain an 1-parameter family S ( t ) of symplectic subspaces which is uniquely determined by 𝜔-decomposition of S . As in Theorem 7, the construction of symplectic subspace S ( t ) is independent of the choice of the 𝜔-basis on S . Therefore we have the following well defined map PPT Slide
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which can be easily seen to be a strong deformation retract from Gr Sp (2 k , 2 n ) to PPT Slide
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. PPT Slide
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Remark. 1. If we vary the complex structure J on V , then the corresponding PPT Slide
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will move inside Gr Sp (2 k , 2 n ) and cover the whole symplectic Grassmannian (see  ).
• 2. ThusandGrSp(2k, 2n) are homotopically equivalent to each other.
• 3. The dimensions of λi-spacesSλiin a 𝜔-decomposition ofSis preserved under deformation.
Acknowledgements
The author would like to thank the referees for their valuable comments which helped to improve the manuscript. This research is supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology(No.2014027205).
References