# Linear algebra identities of inverse matricies

1. Jul 20, 2010

### SpiffyEh

1. The problem statement, all variables and given/known data
Left Inversion in Rectangular Cases. Let A$$^{-1}_{left}$$ = (A$$^{T}$$A)$$^{-1}$$A$$^{T}$$ show A$$^{-1}_{left}$$A = I.

This matrix is called the left-inverse of A and it can be shown that if A $$\in$$ R$$^{m x n}$$ such that A has a pivot in every column then the left inverse exists.

Right Inversion in Rectangular Cases. Let A$$^{-1}_{right}$$ = A$$^{T}$$(AA$$^{T}$$)$$^{-1}$$. Show AA$$^{-1}_{right}$$ = I.

This matrix is called the right-inverse of A and it can be shown that if A $$\in$$ R$$^{m x n}$$ such that A has a pivot in every row then the right inverse exists.

2. Relevant equations

3. The attempt at a solution
I tried the left part and this is what I did:
A$$^{-1}_{left}$$ / (A$$^{T}$$A)$$^{-1}$$= A$$^{T}$$
A$$^{-1}_{left}$$(A$$^{T}$$A) = A$$^{T}$$
A$$^{-1}_{left}$$A = A$$^{T}$$( A$$^{T}$$)$$^{-1}$$ = I

I'm not sure if this is correct or not, so I want to see if I have the right idea. I know that A*A$$^{-1}$$ = I so I thought this would work. Also isn't the right one the exact same thing? Or do I have to do that one a different way? Oh and can someone also explain the concept of left and right inverse. I don't really understand it. Thanks

2. Jul 20, 2010

### SpiffyEh

Sorry it's not showing right at all. I'll try to make it clearer. Al is A.

Attempt:
Al^(-1) / (A^T * A)^(-1) = A^T
Al^(-1) * (A^T * A) = A^T
Al^(-1) * A = A^T * (A^T)^(-1) = I
therefore, Al^(-1) * A = I

Hopefully that made what I was trying to do more clear

3. Jul 20, 2010

### hunt_mat

Don't write divide when dealing with matrices, use the inverse notation.
Write the following to compute the inverse of $$A^{T}A$$
$$(A^{T}A)^{-1}(A^{T}A)=I$$
Multiply on the right by the appropriate stuff to find the expression for the inverse and then use this in the definition of the left inverse. I should come out in the wash.

4. Jul 20, 2010

### SpiffyEh

So, I did the (A$$^{T}$$A)$$^{-1}$$(A$$^{T}$$A) = I
and because of the property AA$$^{-1}$$ = I the left side is I. So, is this proff enough for Aleft? Is Aright basically the same thing then?

5. Jul 21, 2010

### hunt_mat

I was trying to get you to show that
$$(A^{T}A)^{-1}=A^{-1}(A^{T})^{-1}$$
Then use this in the definition of the left inverse to compute that
$$A_{left}^{-1}A=I$$

6. Jul 21, 2010

### HallsofIvy

Are you allowed to assume that $A^{-1}$ and $(A^T)^{-1}$ exist?

7. Jul 21, 2010

### hunt_mat

Hmm, most likely not! my bad...

8. Jul 21, 2010

### hunt_mat

And Spiffy, I think you're done in your proof.

Mat

9. Jul 21, 2010

### SpiffyEh

oh ok, thank you. Would I do the same thing for Aright? I'm paranoid because it seems the same but i'm asked about it as well so I expect it to be different.

10. Jul 21, 2010

### hunt_mat

I think you'll be fine. Nothing to worry about.

Mat