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Dual basis : ウィキペディア英語版
Dual basis
In linear algebra, given a vector space ''V'' with a basis ''B'' of vectors indexed by an index set ''I'' (the cardinality of ''I'' is the dimensionality of ''V''), its dual set is a set ''B'' of vectors in the dual space ''V'' with the same index set ''I'' such that ''B'' and ''B'' form a biorthogonal system. The dual set is always linearly independent but does not necessarily span ''V''. If it does span ''V'', then ''B'' is called the dual basis for the basis ''B''.
Denoting the indexed vector sets as B = \_ and B^ = \_, being biorthogonal means that the elements pair to 1 if the indexes are equal, and to zero otherwise. Symbolically, evaluating a dual vector in ''V'' on a vector in the original space ''V'':
:
v^i(v_j) = \delta^i_j =
\begin
1 & \text i = j\\
0 & \text i \ne j\text
\end

where \delta^i_j is the Kronecker delta symbol.
==A categorical and algebraic construction of the dual space==

Another way to introduce the dual space of a vector space (module) is by introducing it in a categorical sense. To do this, let A be a module defined over the ring R (that is, A is an object in the category R\text\mathbf). Then we define the dual space of A, denoted A^, to be \text_R(A,R), the module formed of all R-linear module homomorphisms from A into R. Note then that we may define a dual to the dual, referred to as the double dual of A, written as A^, and defined as \text_R(A^,R).
To formally construct a basis for the dual space, we shall now restrict our view to the case where F is a finite-dimensional free (left) R-module, where R is a ring of unity. Then, we assume that the set X is a basis for F. From here, we define the Kronecker Delta function \delta_ over the basis X by \delta_=1 if x=y and \delta_=0 if x\ne y. Then the set S = \lbrace f_x:F \to R \; | \; f_x(y)=\delta_ \rbrace describes a linearly independent set with each f_x \in \text_R(F,R). Since F is finite-dimensional, the basis X is of finite cardinality. Then, the set S is a basis to F^\ast and F^\ast is a free (right) R-module.

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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