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{{Stub}} == Introduction == <wikitex include="TeXInclude:Grassmann_manifolds">; 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 [[Wikipedia:Grassmannian|Grassmannian]] </wikitex> == Construction and examples == <wikitex include="TeXInclude:Grassmann_manifolds">; </wikitex> === Construction === <wikitex include="TeXInclude:Grassmann_manifolds">; Let $\F=\Rr ,\Cc , \Hh$ be the real, complex or quaternion field and $V$ a vector space over $\F$ of dimension $n$ and let $k\leq n$. A Grassmannian 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. Grassmannian 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 $O(W^\bot)\times O(W)$. {{beginthm|Theorem|{{cite|Milnor&Stasheff1974}}}} $G_k(V)$ is a Hausdorff, compact, connected smooth manifold of dimension $dk(n-k)$. For $\F=\Cc ,\Hh$ it is also a complex manifold.{{endthm}} Note that the Grassmann manifold $G_k(V)$ around $W\in G_k(V)$ is locally modelled on the vector space $Hom (W^\bot ,W).$ {{beginthm|Proposition|}} There exist a natural diffeomorphism $G_k(V)\simeq G_{n-k}(V^*)$. {{endthm}} </wikitex> === The canonical bundle === <wikitex include="TeXInclude:Grassmann_manifolds">; 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. The tangent bundle to Grassmaniann can be expressed in terms of the canonical bundle: $TG_k(V)= \Hom (\gamma_k^\bot , \gamma_k).$ <wikitex> === Low dimensional Grassmannians === <wikitex include="TeXInclude:Grassmann_manifolds">; 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 $\Oo$, however the higher dimensional projective spaces over octonions do not exist. The maps $p_d :S^{2d-1}\to S^d$ for $d= 2,4,8$ are called the Hopf maps and they play a very important role in homotopy theory; a fibre of $p_d$ is a sphere $S^{d-1}$. <wikitex> === Embeddings of Grassmannians into affine and projective space === <wikitex include="TeXInclude:Grassmann_manifolds">; 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 $G_k(V)$. <wikitex> === Infinite dimensional Grassmannians === <wikitex include="TeXInclude:Grassmann_manifolds">; Infinite dimensional Grassmannians. Natural inclusions of vector space $\F ^1 \subset \F ^2 \subset ...\F ^n \subset ...$ defines inclusions of Grassmannians. The colimit of the resulting sequence is denoted $G_k(\F^{\infty} )$ and also $BGL(k,\F)$. One can also take the colimit with respect to both dimension of the space and of the subspaces. We have a sequence of inclusions $G_1(\F^2)\subset G_2(F^4)\subset ... \subset G_n(F^{2n}) \subset ...$ and its colimit is denoted $B\GL (\F).$ <wikitex> == Invariants == <wikitex include="TeXInclude:Grassmann_manifolds">; <wikitex> === Homotopy groups === <wikitex include="TeXInclude:Grassmann_manifolds">; Homotopy groups of Grassmannians are closely related to homotopy groups of spheres via the appropriate fibration sequences. They also imply that the groups $\pi_i(G_k (V))$ do not depend on $V$, if $k\leq\leq dim V.$ Homotopy groups in the stable range are described by the Bott periodicity theorem. {{beginthm|Proposition|{(R.Bott)}}} For each $i>0$ there are isomorphisms: $\pi_i(BGL(\Rr) \simeq \pi_{i+8}(BGL(\Rr)$ and $\pi_i(BGL(\Cc) \simeq \pi_{i+2}(BGL(\Cc)$ {{endthm}} === Cohomology groups === <wikitex include="TeXInclude:Grassmann_manifolds">; ... </wikitex> == Classification/Characterization == <wikitex include="TeXInclude:Grassmann_manifolds">; ... </wikitex> == Further discussion == <wikitex>; Grassmann manifolds are examples of [[coadjoint orbits]] \cite{Kirillov2004}. </wikitex> == References == {{#RefList:}} ==External links== * The Wikipedia page on [[Wikipedia:Grassmannian|Grassmannian]] <!-- Please modify these headings or choose other headings according to your needs. --> [[Category:Manifolds]]

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