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 of invertible matrices. Left multiplication (pre-multiplication) by an elementary matrix represents elementary row operations, while right multiplication (post-multiplication) represents elementary column operations. The acronym "ERO" is commonly used for "elementary row 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.
In algebraic K-theory, "elementary matrices" refers only to the row-addition matrices.
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.
- 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 the elementary matrix on the left, E⋅A. The elementary matrix for any row operation is obtained by executing the operation on the identity matrix.
Row-switching transformations 
The first type of row operation on a matrix A switches all matrix elements on row i with their counterparts on row j. The corresponding elementary matrix is obtained by swapping row i and row j of the identity matrix.
- So Tij⋅A is the matrix produced by exchanging row i and row j of A.
- The inverse of this matrix is itself: Tij−1=Tij.
- Since the determinant of the identity matrix is unity, det[Tij] = −1. It follows that for any square matrix A (of the correct size), we have det[TijA] = −det[A].
Row-multiplying transformations 
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). The corresponding elementary matrix is a diagonal matrix, with diagonal entries 1 everywhere except in the ith position, where it is m.
- So Ti(m)⋅A is the matrix produced from A by multiplying row i by m.
- The inverse of this matrix is: Ti(m)−1 = Ti(1/m).
- The matrix and its inverse are diagonal matrices.
- det[Ti(m)] = m. Therefore for a square matrix A (of the correct size), we have det[Ti(m)A] = m det[A].
Row-addition transformations 
The final type of row operation on a matrix A adds row j multiplied by a scalar m to row i. The corresponding elementary matrix is the identity matrix but with an m in the (i,j) position.
- So Ti,j(m)⋅A is the matrix produced from A by adding m times row j to row i.
- These transformations are a kind of shear mapping, also known as a transvections.
- Tij(m)−1 = Tij(−m) (inverse matrix).
- The matrix and its inverse are triangular matrices.
- det[Tij(m)] = 1. Therefore, for a square matrix A (of the correct size) we have det[Tij(m)A] = det[A].
- Row-addition transforms satisfy the Steinberg relations.
See also 
- Gaussian elimination
- Linear algebra
- System of linear equations
- Matrix (mathematics)
- LU decomposition
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