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symmetric tensor : ウィキペディア英語版
symmetric tensor
In mathematics, a symmetric tensor is a tensor that is invariant under a permutation of its vector arguments:
:T(v_1,v_2,\dots,v_r) = T(v_,v_,\dots,v_)
for every permutation σ of the symbols .
Alternatively, a symmetric tensor of order ''r'' represented in coordinates as a quantity with ''r'' indices satisfies
:T_ = T_\dots i_}.
The space of symmetric tensors of order ''r'' on a finite-dimensional vector space is naturally isomorphic to the dual of the space of homogeneous polynomials of degree ''r'' on ''V''. Over fields of characteristic zero, the graded vector space of all symmetric tensors can be naturally identified with the symmetric algebra on ''V''. A related concept is that of the antisymmetric tensor or alternating form. Symmetric tensors occur widely in engineering, physics and mathematics.
==Definition==
Let ''V'' be a vector space and
:T\in V^
a tensor of order ''k''. Then ''T'' is a symmetric tensor if
:\tau_\sigma T = T\,
for the braiding maps associated to every permutation σ on the symbols (or equivalently for every transposition on these symbols).
Given a basis of ''V'', any symmetric tensor ''T'' of rank ''k'' can be written as
:T = \sum_^N T_ e^ \otimes e^\otimes\cdots \otimes e^
for some unique list of coefficients T_ (the ''components'' of the tensor in the basis) that are symmetric on the indices. That is to say
:T_\dots i_} = T_
for every permutation σ.
The space of all symmetric tensors of order ''k'' defined on ''V'' is often denoted by ''S''''k''(''V'') or Sym''k''(''V''). It is itself a vector space, and if ''V'' has dimension ''N'' then the dimension of Sym''k''(''V'') is the binomial coefficient
:\dim\, \operatorname^k(V) = .
We then construct Sym(''V'') as the direct sum of Sym''k''(''V'') for ''k'' = 0,1,2,…
:\operatorname(V)= \bigoplus_^\infty \operatorname^k(V).

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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