# Prove that the matrices have the same rank.

1. Oct 2, 2009

### Dafe

1. The problem statement, all variables and given/known data
Prove that the three matrices have the same rank.

$$\left[ \begin{array}{c} A\\ \end{array} \right]$$

$$\left[ \begin{array}{c} A & A\\ \end{array} \right]$$

$$\left[ \begin{array}{cc} A & A\\ A & A\\ \end{array} \right]$$

2. Relevant equations

3. The attempt at a solution

If elimination is done on the second matrix it will become:
$$\left[ \begin{array}{c} A & 0\\ \end{array} \right]$$

This means that the rank is still the same as A.

Elimination on the third matrix gives:
$$\left[ \begin{array}{cc} A & A\\ 0 & 0\\ \end{array} \right]$$

Since no new independent vectors are added, it also has rank A.

Thanks.

2. Oct 2, 2009

### Billy Bob

About the only thing different I would say is to suppose that when
$$\left[ \begin{array}{c} A\\ \end{array} \right]$$
is reduced you obtain
$$\left[ \begin{array}{c} R\\ \end{array} \right]$$

Then express your reduced forms of the other two matrices in terms of R instead of A. Refer to the number of nonzero rows in R, and you are done.

3. Oct 4, 2009

### Dafe

Hi Billy Bob, thanks for the reply.
Here's the way I think you would do it: (just showing one matrix)

$$\left[ \begin{array}{c} A & A\\ \end{array} \right]$$

$$\left[ \begin{array}{c} R & 0\\ \end{array} \right]$$

# non zero rows = r for all matrices.