Grassmann manifolds

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1 Introduction

Grassmann manifolds are named after Hermann Grassmann, a German school teacher in Stettin who developed basic notions of linear algebra. Let $\newcommand{\Z}{\mathbb{Z}} \newcommand{\R}{\mathbb{R}} \newcommand{\C}{\mathbb{C}} \newcommand{\N}{\mathbb{N}} \newcommand{\Q}{\mathbb{Q}} \newcommand{\F}{\mathbb{F}} \newcommand{\bZ}{\mathbb{Z}} \newcommand{\bR}{\mathbb{R}} \newcommand{\bC}{\mathbb{C}} \newcommand{\bH}{\mathbb{H}} \newcommand{\bQ}{\mathbb{Q}} \newcommand{\bF}{\mathbb{F}} \newcommand{\bN}{\mathbb{N}} \DeclareMathOperator\id{id} % identity map \DeclareMathOperator\Sq{Sq} % Steenrod squares \DeclareMathOperator\Homeo{Homeo} % group of homeomorphisms of a topoloical space \DeclareMathOperator\Diff{Diff} % group of diffeomorphisms of a smooth manifold \DeclareMathOperator\SDiff{SDiff} % diffeomorphism under some constraint \DeclareMathOperator\Hom{Hom} % homomrphism group \DeclareMathOperator\End{End} % endomorphism group \DeclareMathOperator\Aut{Aut} % automorphism group \DeclareMathOperator\Inn{Inn} % inner automorphisms \DeclareMathOperator\Out{Out} % outer automorphism group \DeclareMathOperator\vol{vol} % volume \newcommand{\GL}{\text{GL}} % general linear group \newcommand{\SL}{\text{SL}} % special linear group \newcommand{\SO}{\text{SO}} % special orthogonal group \newcommand{\O}{\text{O}} % orthogonal group \newcommand{\SU}{\text{SU}} % special unitary group \newcommand{\Spin}{\text{Spin}} % Spin group \newcommand{\RP}{\Rr\mathrm P} % real projective space \newcommand{\CP}{\Cc\mathrm P} % complex projective space \newcommand{\HP}{\Hh\mathrm P} % quaternionic projective space \newcommand{\Top}{\mathrm{Top}} % topological category \newcommand{\PL}{\mathrm{PL}} % piecewise linear category \newcommand{\Cat}{\mathrm{Cat}} % any category \newcommand{\KS}{\text{KS}} % Kirby-Siebenmann class \newcommand{\Hud}{\text{Hud}} % Hudson torus \newcommand{\underbar}{\underline} %Classifying Spaces for Families of Subgroups \newcommand{\textup}{\text} \newcommand{\sp}{^}\newcommand{\F}{\mathbb{F}}\F$ be the real, complex or quaternion field and $V$ a vector space over $\F$ of dimension $n$ and let $k\leq n$ . A Grassmann manifolds of $k$ -dimensional subspaces is a set $G_k(V)$ of $k$ -dimensional subspaces. The set $G_k(V)$ is a quotient of a subset of $V\times ...\times V$ consisting of linearly independent $k$ -tuples of vectors with the subspace topology. We define topology on $G_k(V)$ as the quotient topology.

Theorem 1.1 [Milnor&Stasheff1974]. $G_k(V)/var/www/vhost/map.mpim-bonn.mpg.de/tmp/AppWikiTex/tex_iyBvEK$ $G_k(V)$ is a Hausdorff, compact space.

Theorem 1.2 [Milnor&Stasheff1974]. $G_k(V)/var/www/vhost/map.mpim-bonn.mpg.de/tmp/AppWikiTex/tex_1GztGi$ $G_k(V)$ is a connected, compact smooth manifold of dimension $dk(n-k)/var/www/vhost/map.mpim-bonn.mpg.de/tmp/AppWikiTex/tex_Epg66Q$ $dk(n-k)$ . For

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$\F=\C,\H$ it is also a complex manifold.

Note that the Grassmann manifold $G_k(V)$ around $W\in G_k(V)$ is locally modelled on the vector space $Hom (W^\bot ,W).$ </div>

Grassmann manifold is a homogeneous space of the general linear group. General linear group $GL(V)$ acts transitively on $G_k(V)$ with an isotropy group consisting of automorphisms preserving a given subspace. If the space $V$ is equipped with a scalar product (hermitian metric resp.) then the group of isometries $O(V)$ acts transitively and the isotropy group of $W$ is $O(W^\bot)\times O(W)$ .

The Grassmann manifold is equipped with the canonical, tautological vector bundle $\gamma^V_k.$ which is a subbundle of the trivial bundle $G_k(V)\times V \to G_k(V)\times V$ . The total space is $E(\gamma^V_k) = \{(W,w)\in G_k(V)\times V\, |\,\, w\in W \}$ The total space of the associated principal bundle is a Stiefel manifold.

Proposition 1.3. There exist a natural diffeomorphism $G_k(V)\simeq G_{n-k}(V^*)/var/www/vhost/map.mpim-bonn.mpg.de/tmp/AppWikiTex/tex_Azg5ni$ $G_k(V)\simeq G_{n-k}(V^*)$ .

The Grassmannians $G_1(V)$ are projective spaces, denoted $P (V)$ . Note that $G_1(F^2)=S^d$ , where $d=dim_R F$ . If we identify $S^d$ with the one-point compactification of $\F$ the projection of the canonical principal bundle corresponds to the map $p_d :S^{2d-1}\to S^d$ given by $p_d(z_0,z_1)=z_0/z_1$ where $z_i\in\F$ . Note, that the same formula works for octonions, however the higher dimensional projective spaces over octonions do not exist. The maps $p_d :S^{2d-1}\to S^d$ for $d=1,2,4,8$ are called the Hopf maps and they play a very important role in homotopy theory; a fiber of $p_d$ is a sphere $S^{d-1}$ .

There is an embedding of the Grassmannian  $G_k(V)$  in the cartesian space  $\F^{n^2}=\Hom\,(F^n,F^n)$  which assigns to every subspace the orthogonal projection on it. If  $V$  is equipped with a norm, the embedding defines a natural (operator) metric on  $G_k(V)$ .

Prove that there is a free action of the group $O(k,\F)$ on $V_k(\F^n)$ sucht that the orbit space is homeomorphic to $G_k(\F^n)$ . Similarly for the noncompact Stiefel manifold. \end{zad}

Prove that the map $p:V_k(\F^n)\to G_k(\F^n)$ is locally trivial (even a principal $O(k,\F)$ -bundle), thus a fibration.

2 Construction and examples

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3 Invariants

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4 Classification/Characterization

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5 Further discussion

Grassmann manifolds are examples of coadjoint orbits [Kirillov2004].

6 References

[Kirillov2004] A. A. Kirillov, Lectures on the orbit method, Graduate Studies in Mathematics 64, American Mathematical Society, Providence, RI, 2004. MR2069175 (2005c:22001) Zbl 02121486
[Milnor&Stasheff1974] J. W. Milnor and J. D. Stasheff, Characteristic classes, Princeton University Press, Princeton, N. J., 1974. MR0440554 (55 #13428) Zbl 1079.57504

Grassmann manifolds

Contents

1 Introduction

2 Construction and examples

3 Invariants

4 Classification/Characterization

5 Further discussion

6 References

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