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In mathematics, the quasideterminant is a replacement for the determinant for matrices with noncommutative entries. Example 2 × 2 quasideterminants are as follows: : 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, : means delete the ''i''th row and ''j''th column from ''A''. The 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 . 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 is built from by rescaling its -th row (on the left) by , then . Similarly, if is built from by adding a (left) multiple of the -th row to another row, then . They even develop a quasideterminantal version of Cramer's rule. ==Definition== Let be an matrix over a (not necessarily commutative) ring and fix . Let denote the ()-entry of , let denote the -th row of with column deleted, and let denote the -th column of with row deleted. The ()-quasideterminant of is defined if the submatrix is invertible over . In this case, :: Recall the formula (for commutative rings) relating to the determinant, namely . The above definition is a generalization in that (even for noncommutative rings) one has :: 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)』 ■ウィキペディアで「quasideterminant」の詳細全文を読む スポンサード リンク
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