# Grassmann manifolds

## 1 Introduction

Grassmann manifolds are named after Hermann Grassmann, a German school teacher in Stettin who developed basic notions of linear algebra. They play a key role in topology and geometry as the universal spaces of vector bundles. See also Grassmannian

## 2 Construction and examples

### 2.1 Construction

Let $\F=\Rr ,\Cc , \Hh$$\newcommand{\Z}{\mathbb{Z}} \newcommand{\R}{\mathbb{R}} \newcommand{\C}{\mathbb{C}} \newcommand{\N}{\mathbb{N}} \newcommand{\Q}{\mathbb{Q}} \newcommand{\F}{\mathbb{F}} \newcommand{\bZ}{\mathbb{Z}} \newcommand{\bR}{\mathbb{R}} \newcommand{\bC}{\mathbb{C}} \newcommand{\bH}{\mathbb{H}} \newcommand{\bQ}{\mathbb{Q}} \newcommand{\bF}{\mathbb{F}} \newcommand{\bN}{\mathbb{N}} \DeclareMathOperator\id{id} % identity map \DeclareMathOperator\Sq{Sq} % Steenrod squares \DeclareMathOperator\Homeo{Homeo} % group of homeomorphisms of a topoloical space \DeclareMathOperator\Diff{Diff} % group of diffeomorphisms of a smooth manifold \DeclareMathOperator\SDiff{SDiff} % diffeomorphism under some constraint \DeclareMathOperator\Hom{Hom} % homomrphism group \DeclareMathOperator\End{End} % endomorphism group \DeclareMathOperator\Aut{Aut} % automorphism group \DeclareMathOperator\Inn{Inn} % inner automorphisms \DeclareMathOperator\Out{Out} % outer automorphism group \DeclareMathOperator\vol{vol} % volume \newcommand{\GL}{\text{GL}} % general linear group \newcommand{\SL}{\text{SL}} % special linear group \newcommand{\SO}{\text{SO}} % special orthogonal group \newcommand{\O}{\text{O}} % orthogonal group \newcommand{\SU}{\text{SU}} % special unitary group \newcommand{\Spin}{\text{Spin}} % Spin group \newcommand{\RP}{\Rr\mathrm P} % real projective space \newcommand{\CP}{\Cc\mathrm P} % complex projective space \newcommand{\HP}{\Hh\mathrm P} % quaternionic projective space \newcommand{\Top}{\mathrm{Top}} % topological category \newcommand{\PL}{\mathrm{PL}} % piecewise linear category \newcommand{\Cat}{\mathrm{Cat}} % any category \newcommand{\KS}{\text{KS}} % Kirby-Siebenmann class \newcommand{\Hud}{\text{Hud}} % Hudson torus \newcommand{\Ker}{\text{Ker}} % Kernel \newcommand{\underbar}{\underline} %Classifying Spaces for Families of Subgroups \newcommand{\textup}{\text} \newcommand{\sp}{^}\newcommand{\F}{\mathbb{F}}\F=\Rr ,\Cc , \Hh$ be the real, complex or quaternion field and $V$$V$ a vector space over $\F$$\F$ of dimension $n$$n$ and let $k\leq n$$k\leq n$. A Grassmannian of $k$$k$-dimensional subspaces is a set $G_k(V)$$G_k(V)$ of $k$$k$-dimensional subspaces. The set $G_k(V)$$G_k(V)$ is a quotient of a subset of $V\times ...\times V$$V\times ...\times V$ consisting of linearly independent $k$$k$-tuples of vectors with the subspace topology. We define topology on $G_k(V)$$G_k(V)$ as the quotient topology. Grassmannian is a homogeneous space of the general linear group. General linear group $\GL(V)$$\GL(V)$ acts transitively on $G_k(V)$$G_k(V)$ with an isotropy group consisting of automorphisms preserving a given subspace. If the space $V$$V$ is equipped with a scalar product (hermitian metric resp.) then the group of isometries $O(V)$$O(V)$ acts transitively and the isotropy group of $W$$W$ is $O(W^\bot)\times O(W)$$O(W^\bot)\times O(W)$.

Theorem 2.1 [Milnor&Stasheff1974]. $G_k(V)$$G_k(V)$ is a Hausdorff, compact, connected smooth manifold of dimension $dk(n-k)$$dk(n-k)$. For $\F=\Cc ,\Hh$$\F=\Cc ,\Hh$ it is also a complex manifold.

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

Proposition 2.2. There exist a natural diffeomorphism $G_k(V)\simeq G_{n-k}(V^*)$$G_k(V)\simeq G_{n-k}(V^*)$.

### 2.2 The canonical bundle

The Grassmann manifold is equipped with the canonical, tautological vector bundle $\gamma^V_k.$$\gamma^V_k.$ which is a subbundle of the trivial bundle $G_k(V)\times V \to G_k(V)\times V$$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 \}$$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.

The tangent bundle to Grassmaniann can be expressed in terms of the canonical bundle: $TG_k(V)= \Hom (\gamma_k^\bot , \gamma_k).$$TG_k(V)= \Hom (\gamma_k^\bot , \gamma_k).$

### 2.3 Low dimensional Grassmannians

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

### 2.4 Embeddings of Grassmannians into affine and projective space

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

### 2.5 Infinite dimensional Grassmannians

Infinite dimensional Grassmannians. Natural inclusions of vector space $\F ^1 \subset \F ^2 \subset ...\F ^n \subset ...$$\F ^1 \subset \F ^2 \subset ...\F ^n \subset ...$ defines inclusions of Grassmannians. The colimit of the resulting sequence is denoted $G_k(\F^{\infty} )$$G_k(\F^{\infty} )$ and also $BGL(k,\F)$$BGL(k,\F)$. One can also take the colimit with respect to both dimension of the space and of the subspaces. We have a sequence of inclusions $G_1(\F^2)\subset G_2(\F^4)\subset ... \subset G_n(\F^{2n}) \subset ...$$G_1(\F^2)\subset G_2(\F^4)\subset ... \subset G_n(\F^{2n}) \subset ...$ and its colimit is denoted $B\GL (\F).$$B\GL (\F).$

## 3 Invariants

### 3.1 Homotopy groups

Homotopy groups of Grassmannians are closely related to homotopy groups of spheres via the appropriate fibration sequences. They also imply that the groups $\pi_i(G_k (V))$$\pi_i(G_k (V))$ do not depend on $V$$V$, if $k\leq\leq dim V.$$k\leq\leq dim V.$ Homotopy groups in the stable range are described by the Bott periodicity theorem.

Proposition 3.1 {(R.Bott).} For each $i>0$$i>0$ there are isomorphisms: $\pi_i(BGL(\Rr)) \simeq \pi_{i+8}(BGL(\Rr))$$\pi_i(BGL(\Rr)) \simeq \pi_{i+8}(BGL(\Rr))$ and $\pi_i(BGL(\Cc)) \simeq \pi_{i+2}(BGL(\Cc))$$\pi_i(BGL(\Cc)) \simeq \pi_{i+2}(BGL(\Cc))$

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## 5 Further discussion

Grassmann manifolds are examples of coadjoint orbits [Kirillov2004].