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quasideterminant : ウィキペディア英語版
quasideterminant
In mathematics, the quasideterminant is a replacement for the determinant for matrices with noncommutative entries. Example 2 × 2 quasideterminants are as follows:
:
\left|\begin
a_ & a_ \\
a_ & a_ \end
\right|_ = a_ - a_a_
\qquad
\left|\begin
a_ & a_ \\
a_ & a_ \end
\right|_ = a_ - a_a_.

In general, there are ''n''2 quasideterminants defined for an ''n'' × ''n'' matrix (one for each position in the matrix), but the presence of the inverted terms above should give the reader pause: they are not always defined, and even when they are defined, they do not reduce to determinants when the entries commute. Rather,
:
\left|A\right|_ = (-1)^ \frac means delete the ''i''th row and ''j''th column from ''A''.
The 2\times2 examples above were introduced between 1926 and 1928 by Richardson
〔A.R. Richardson, Hypercomplex determinants, ''Messenger of Math.'' 55 (1926), no. 1.〕
〔A.R. Richardson, Simultaneous linear equations over a division algebra, ''Proc. London Math. Soc.'' 28 (1928), no. 2.〕 and Heyting,
〔A. Heyting, Die theorie der linearen gleichungen in einer zahlenspezies mit nichtkommutativer multiplikation, ''Math. Ann. 98'' (1928), no. 1.〕
but they were marginalized at the time because they were not polynomials in the entries of A. These examples were rediscovered and given new life in 1991 by I.M. Gelfand and V.S. Retakh.
〔I. Gelfand, V. Retakh, Determinants of matrices over noncommutative rings, ''Funct. Anal. Appl.'' 25 (1991), no. 2.〕
〔I. Gelfand, V. Retakh, Theory of noncommutative determinants, and characteristic functions of graphs, ''Funct. Anal. Appl.'' 26 (1992), no. 4.〕
There, they develop quasideterminantal versions of many familiar determinantal properties. For example, if B is built from A by rescaling its i-th row (on the left) by \left.\rho\right., then \left|B\right|_ = \rho \left|A\right|_.
Similarly, if B is built from A by adding a (left) multiple of the k-th row to another row, then \left|B\right|_ = \left|A\right|_ \,\, (\forall j; \forall k\neq i). They even develop a quasideterminantal
version of Cramer's rule.
==Definition==

Let A be an n\times n matrix over a (not necessarily commutative)
ring R and fix 1\leq i,j\leq n. Let a_
denote the (i,j)-entry of A, let r_i^j denote the i-th row of A with column j deleted, and let c_j^i denote the j-th column of A with row i deleted. The (i,j)-quasideterminant of A is defined if the submatrix A^ is invertible over R. In this case,
::
\left|A\right|_ = a_ - r_i^j\, \bigl(A^\bigr)^\, c_j^i .

Recall the formula (for commutative rings) relating A^ to the determinant, namely (A^)_ = (-1)^ \frac. The above definition is a generalization in that (even for noncommutative rings) one has
::
\bigl(A^\bigr)_ = \left|A\right|_^

whenever the two sides makes sense.

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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