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Jordan matrix : ウィキペディア英語版
Jordan matrix
In the mathematical discipline of matrix theory, a Jordan block over a ring R (whose identities are the zero 0 and one 1) is a matrix composed of 0 elements everywhere except for the diagonal, which is filled with a fixed element \lambda\in R, and for the superdiagonal, which is composed of ones. The concept is named after Camille Jordan.
:\begin
\lambda & 1 & 0 & \cdots & 0 \\
0 & \lambda & 1 & \cdots & 0 \\
\vdots & \vdots & \vdots& \ddots & \vdots \\
0 & 0 & 0 & \lambda & 1 \\
0 & 0 & 0 & 0 & \lambda \end
Every Jordan block is thus specified by its dimension ''n'' and its eigenvalue \lambda and is indicated as J_.
Any block diagonal matrix whose blocks are Jordan blocks is called a Jordan matrix; using either the \oplus or the “\mathrm” symbol, the (l+m+n)\times (l+m+n) block diagonal square matrix whose first diagonal block is J_, whose second diagonal block is J_ and whose third diagonal block is J_ is compactly indicated as J_\oplus J_\oplus J_ or \mathrm\left(J_, J_, J_\right), respectively.
For example the matrix
:
J=\left(\begin
0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & i & 1 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & i & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & i & 1 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & i & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 7 & 1 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 7 & 1 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 7 \end\right)
is a 10\times 10 Jordan matrix with a 3\times 3 block with eigenvalue 0, two 2\times 2 blocks with eigenvalue the imaginary unit and a 3\times 3 block with eigenvalue 7. Its Jordan-block structure can also be written as either J_\oplus J_\oplus J_\oplus J_ or \mathrm\left(J_,J_,J_,J_\right).
== Linear algebra ==
Any n\times n square matrix A whose elements are in an algebraically closed field K is similar to a Jordan matrix J, also in \mathbb_n (K), which is unique up to a permutation of its diagonal blocks themselves. J is called the Jordan normal form of A and corresponds to a generalization of the diagonalization procedure. A diagonalizable matrix is similar, in fact, to a special case of Jordan matrix: the matrix whose blocks are all 1\times 1.
More generally, given a Jordan matrix J=J_\oplus J_ \oplus\ldots\oplus J_, i.e. whose k^\text diagonal block, 1\leq k\leq N is the Jordan block J_ and whose diagonal elements \lambda_k may not all be distinct, the geometric multiplicity of \lambda\in K for the matrix J, indicated as \mathrm_J \lambda\,, corresponds to the number of Jordan blocks whose eigenvalue is \lambda. Whereas the index of an eigenvalue \lambda for J, indicated as \mathrm_J \lambda\,, is defined as the dimension of the largest Jordan block associated to that eigenvalue.
The same goes for all the matrices A similar to J, so \mathrm_A \lambda\, can be defined accordingly with respect to the Jordan normal form of A for any of its eigenvalues \lambda \in\mathrmA. In this case one can check that the index of \lambda for A is equal to its multiplicity as a root of the minimal polynomial of A (whereas, by definition, its algebraic multiplicity for A, \mathrm_A \lambda\,, is its multiplicity as a root of the characteristic polynomial of A, i.e. \det(A-xI)\in K()).
An equivalent necessary and sufficient condition for A to be diagonalizable in K is that all of its eigenvalues have index equal to 1, i.e. its minimal polynomial has only simple roots.
Note that knowing a matrix's spectrum with all of its algebraic/geometric multiplicities and indexes does not always allow for the computation of its Jordan normal form (this may be a sufficient condition only for spectrally simple, usually low-dimensional matrices): the Jordan decomposition is, in general, a computationally challenging task.
From the vector space point of view, the Jordan decomposition is equivalent to finding an orthogonal decomposition (i.e. via direct sums of eigenspaces represented by Jordan blocks) of the domain which the associated generalized eigenvectors make a basis for.

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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