# The Unapologetic Mathematician

## Elementary Matrices Generate the General Linear Group

Okay, so we can use elementary row operations to put any matrix into its (unique) reduced row echelon form. As we stated last time, this consists of building up a basis for the image of the transformation the matrix describes by walking through a basis for the domain space and either adding a new, independent basis vector or writing the image of a domain basis vector in terms of the existing image basis vectors.

So let’s say we’ve got a transformation $A$ in $\mathrm{GL}(n,\mathbb{F})$. Given a basis, we get an invertible matrix (which we’ll also call $A$). Then we can use elementary row operations to put this matrix into its reduced row echelon form. But now every basis vector gets sent to a vector that’s linearly independent of all the others, or else the transformation $A$ wouldn’t be invertible! That is, the reduced row echelon form of the matrix $A$ must be the identity matrix.

But remember that every one of our elementary row operations is the result of multiplying on the left by an elementary matrix. So we can take the matrices corresponding to the list of all the elementary row operations and write

$\displaystyle\left(E_kE_{k-1}\dots E_2E_1\right)A=I$

which tells us that applying all these elementary row operations one after another leads us to the identity matrix. But this means that the product of all the elementary matrices on the right is $A^{-1}$. And since we can also apply this to the transformation $A^{-1}$, we can find a list of elementary matrices whose product is $A$. That is, any invertible linear transformation can be written as the product of a finite list of elementary matrices, and thus the elementary matrices generate the general linear group.

September 4, 2009 - Posted by | Algebra, Linear Algebra

1. […] that if we restrict to upper shears we can generate all upper-unipotent matrices. On the other hand if we use all shears and scalings we can generate any invertible matrix we want (since swaps can be built from shears and scalings). […]

Pingback by Shears Generate the Special Linear Group « The Unapologetic Mathematician | September 9, 2009 | Reply

2. HI!
Very helpful reading this. But how do you prove that you only need the first and third kind of elementary matrices to generate the general linear group?

Comment by Nicolas | October 5, 2010 | Reply

• I think I’ll leave that as an exercise: write an arbitrary matrix of the second kind in terms of matrices of the first and third kinds,

Comment by John Armstrong | October 5, 2010 | Reply

3. […] all invertible matrices can be written as a product of elementary matrices of the first and third kind, we just have to show that we can row-reduced into the identity using shears before scales, […]

Pingback by Scales and Shears are (Sort of) Commutative and the Latter Generate the Special Linear Group « indefiniteintegirl | October 26, 2011 | Reply