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Vectorization (mathematics) : ウィキペディア英語版
Vectorization (mathematics)
In mathematics, especially in linear algebra and matrix theory, the vectorization of a matrix is a linear transformation which converts the matrix into a column vector. Specifically, the vectorization of an ''m×n'' matrix ''A'', denoted by vec(''A''), is the ''mn'' × 1 column vector obtained by stacking the columns of the matrix ''A'' on top of one another:
:\mathrm(A) = (\ldots, a_, a_, \ldots, a_, \ldots, a_, \ldots, a_ )^T
Here a_ represents the (i,j)-th element of matrix A and the superscript ^T denotes the transpose. Vectorization expresses the isomorphism \mathbf^ := \mathbf^m \otimes \mathbf^n \cong \mathbf^ between these vector spaces (of matrices and vectors) in coordinates.
For example, for the 2×2 matrix A = \begin a & b \\ c & d \end, the vectorization is \mathrm(A) = \begin a \\ c \\ b \\ d \end.
==Compatibility with Kronecker products==

The vectorization is frequently used together with the Kronecker product to express matrix multiplication as a linear transformation on matrices. In particular,
: \mbox(ABC)=(C^\otimes A)\mbox(B)
for matrices ''A'', ''B'', and ''C'' of dimensions ''k×l'', ''l×m'', and ''m×n''. For example, if \mbox_A(X) = AX-XA (the adjoint endomorphism of the Lie algebra gl(''n'',C) of all ''n×n'' matrices with complex entries), then \mbox(\mbox_A(X)) = (I_n\otimes A - A^T \otimes I_n ) \mbox(X), where I_n is the ''n×n'' identity matrix.
There are two other useful formulations:
: \mbox(ABC)=(I_n\otimes AB)\mbox(C) =(C^B^\otimes I_k)\mbox(A)
: \mbox(AB)=(I_m\otimes A)\mbox(B) =(B^\otimes I_k)\mbox(A)

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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