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Sylvester's determinant theorem : ウィキペディア英語版
Sylvester's determinant theorem
In matrix theory, Sylvester's determinant theorem is a theorem useful for evaluating certain types of determinants. It is named after James Joseph Sylvester, who stated this theorem without proof in 1851.〔
Cited in 〕
The theorem states that if ''A'', ''B'' are matrices of size ''p'' × ''n'' and ''n'' × ''p'' respectively, then
:\det(I_p + AB) = \det(I_n + BA),\
where ''I''''a'' is the identity matrix of order ''a''.〔 page 416〕
This can be seen for invertible ''A'', ''B'' by conjugating ''I + AB'' by ''A−1'', then extended to arbitrary square matrices by density of invertible matrices, and then to arbitrary
rectangular matrices by adding zero column or row vectors as necessary.
It is closely related to the Matrix determinant lemma and its generalization. It is the determinant analogue of the Woodbury matrix identity for matrix inverses.
==Proof==
The theorem may be proven as follows.〔.〕 Let M be a matrix comprising the four blocks -A, B, I_n and I_p
:M = \beginI_p & -A \\ B & I_n \end .
Block LU decomposition of M yields
:M = \beginI_p & 0 \\ B & I_n \end \beginI_p & -A \\ 0 & I_n + B A \end
from which
:\det(M) = \det(I_n + B A)
follows. Decomposing M to an upper and a lower triangular matrix instead,
:M = \beginI_p + A B & -A \\ 0 & I_n \end \beginI_p & 0 \\ B & I_n \end,
yields
:\det(M) = \det(I_p + A B).
This proves
:\det(I_n + B A) = \det(I_p + A B).

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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