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Cauchy–Binet formula : ウィキペディア英語版
Cauchy–Binet formula
In linear algebra, the Cauchy–Binet formula, named after Augustin-Louis Cauchy and Jacques Philippe Marie Binet, is an identity for the determinant of the product of two rectangular matrices of transpose shapes (so that the product is well-defined and square). It generalizes the statement that the determinant of a product of square matrices is equal to the product of their determinants. The formula is valid for matrices with entries from any commutative ring.
== Statement ==

Let ''A'' be an ''m''×''n'' matrix and ''B'' an ''n''×''m'' matrix. Write () for the set , and \tbinomm for the set of ''m''-combinations of () (i.e., subsets of size ''m''; there are \tbinom nm of them). For S\in\tbinomm, write ''A''(),''S'' for the ''m''×''m'' matrix whose columns are the columns of ''A'' at indices from ''S'', and ''B''''S'',() for the ''m''×''m'' matrix whose rows are the rows of ''B'' at indices from ''S''. The Cauchy–Binet formula then states
: \det(AB) = \sum_ \det(A_)\det(B_).
Example: taking ''m'' = 2 and ''n'' = 3, and matrices A = \begin1&1&2\\
3& 1& -1\\
\end
and B =
\begin1&1\\3&1\\0&2\end, the Cauchy–Binet formula gives the determinant:
:
\det(AB)=
\left|\begin1&1\\3&1\end\right|
\cdot
\left|\begin1&1\\3&1\end\right|
+
\left|\begin1&2\\1&-1\end\right|
\cdot
\left|\begin3&1\\0&2\end\right|
+
\left|\begin1&2\\3&-1\end\right|
\cdot
\left|\begin1&1\\0&2\end\right|.

Indeed AB =\begin4&6\\6&2\end, and its determinant is −28, which is also the value -2\times-2+-3\times6+-7\times2 given by the right hand side of the formula.

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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