Concept

# Matrice élémentaire

Résumé
In mathematics, an elementary matrix is a matrix which differs from the identity matrix by one single elementary row operation. The elementary matrices generate the general linear group GLn(F) when F is a field. Left multiplication (pre-multiplication) by an elementary matrix represents elementary row operations, while right multiplication (post-multiplication) represents elementary column operations. Elementary row operations are used in Gaussian elimination to reduce a matrix to row echelon form. They are also used in Gauss–Jordan elimination to further reduce the matrix to reduced row echelon form. There are three types of elementary matrices, which correspond to three types of row operations (respectively, column operations): Row switching A row within the matrix can be switched with another row. Row multiplication Each element in a row can be multiplied by a non-zero constant. It is also known as scaling a row. Row addition A row can be replaced by the sum of that row and a multiple of another row. If E is an elementary matrix, as described below, to apply the elementary row operation to a matrix A, one multiplies A by the elementary matrix on the left, EA. The elementary matrix for any row operation is obtained by executing the operation on the identity matrix. This fact can be understood as an instance of the Yoneda lemma applied to the category of matrices. Permutation matrix The first type of row operation on a matrix A switches all matrix elements on row i with their counterparts on a different row j. The corresponding elementary matrix is obtained by swapping row i and row j of the identity matrix. So Ti,j A is the matrix produced by exchanging row i and row j of A. Coefficient wise, the matrix Ti,j is defined by : The inverse of this matrix is itself: Since the determinant of the identity matrix is unity, It follows that for any square matrix A (of the correct size), we have For theoretical considerations, the row-switching transformation can be obtained from row-addition and row-multiplication transformations introduced below because The next type of row operation on a matrix A multiplies all elements on row i by m where m is a non-zero scalar (usually a real number).
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