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Schur–Weyl duality : ウィキペディア英語版
Schur–Weyl duality
Schur–Weyl duality is a mathematical theorem in representation theory that relates irreducible finite-dimensional representations of the general linear and symmetric groups. It is named after two pioneers of representation theory of Lie groups, Issai Schur, who discovered the phenomenon, and Hermann Weyl, who popularized it in his books on quantum mechanics and classical groups as a way of classifying representations of unitary and general linear groups.
== Description ==
Schur–Weyl duality forms an archetypical situation in representation theory involving two kinds of symmetry that determine each other. Consider the tensor space
: \mathbb^n\otimes\mathbb^n\otimes\cdots\otimes\mathbb^n with ''k'' factors.
The symmetric group ''S''''k'' on ''k'' letters acts on this space (on the left) by permuting the factors,
: \sigma(v_1\otimes v_2\otimes\cdots\otimes v_k) = v_\otimes v_\otimes\cdots\otimes v_.
The general linear group ''GL''''n'' of invertible ''n''×''n'' matrices acts on it by the simultaneous matrix multiplication,
: g(v_1\otimes v_2\otimes\cdots\otimes v_k) = gv_1\otimes gv_2\otimes\cdots\otimes gv_k, \quad g\in GL_n.
These two actions commute, and in its concrete form, the Schur–Weyl duality asserts that under the joint action of the groups ''S''''k'' and ''GL''''n'', the tensor space decomposes into a direct sum of tensor products of irreducible modules for these two groups that determine each other,
: \mathbb^n\otimes\mathbb^n\otimes\cdots\otimes\mathbb^n = \sum_D \pi_k^D\otimes\rho_n^D.
The summands are indexed by the Young diagrams ''D'' with ''k'' boxes and at most ''n'' rows, and representations \pi_k^D of ''S''''k'' with different ''D'' are mutually non-isomorphic, and the same is true for representations \rho_n^D of ''GL''''n''.
The abstract form of the Schur–Weyl duality asserts that two algebras of operators on the tensor space generated by the actions of ''GL''''n'' and ''S''''k'' are the full mutual centralizers in the algebra of the endomorphisms \mathrm_\mathbb(\mathbb^n\otimes\mathbb^n\otimes\cdots\otimes\mathbb^n).

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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