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Classifying space for U(n) : ウィキペディア英語版
Classifying space for U(n)
In mathematics, the classifying space for the unitary group U(''n'') is a space BU(''n'') together with a universal bundle EU(''n'') such that any hermitian bundle on a paracompact space ''X'' is the pull-back of EU(''n'') by a map ''X'' → BU(''n'') unique up to homotopy.
This space with its universal fibration may be constructed as either
# the Grassmannian of ''n''-planes in an infinite-dimensional complex Hilbert space; or,
# the direct limit, with the induced topology, of Grassmannians of ''n'' planes.
Both constructions are detailed here.
==Construction as an infinite Grassmannian==
The total space EU(''n'') of the universal bundle is given by
:EU(n)=\left \ \right \}.
Here, ''H'' is an infinite-dimensional complex Hilbert space, the ''e''''i'' are vectors in ''H'', and \delta_ is the Kronecker delta. The symbol (\cdot,\cdot) is the inner product on ''H''. Thus, we have that EU(''n'') is the space of orthonormal ''n''-frames in ''H''.
The group action of U(''n'') on this space is the natural one. The base space is then
:BU(n)=EU(n)/U(n)
and is the set of Grassmannian ''n''-dimensional subspaces (or ''n''-planes) in ''H''. That is,
:BU(n) = \
so that ''V'' is an ''n''-dimensional vector space.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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