# Linear Algebra Question

1. Oct 9, 2008

### qwerty5

1. The problem statement, all variables and given/known data
Write the general element in terms of aij and bij for (AB)^T [AB transposed].

2. Relevant equations

(AB)^T = B^T*A^T; A=[aij]mxn; B=[bij]nxp

3. The attempt at a solution
n
AB= [sigma aik*bkj]mxp. Let this be equal to [xij]mxp
k=1
n
(AB)^T=[[sigma aik*bkj]mxp]^T
k=1

=[xji]pxm

n
=[sigma aki*bjk]mxp
k=1
n
so the general element xji=[sigma aki*bjk]
k=1

My teacher says this is wrong. Where did I go wrong?

------------------------
Alternate way I used to "check" my wrong answer:
n
(AB)^T=B^T*A^T=[bji]pxn[aji]nxm=[sigma bjk*aki]pxm=[sigma aki*bjk]pxm
k=1

We are using an differential equations/linear algebra textbook for engineers. It never discusses element-by-element proofs, and it leaves out many important differential equations topics, such as exact equations. I have a real diff eq book that my neighbor lent me, but I have to teach myself these types of problems through Wikipedia.

2. Oct 9, 2008

### Kreizhn

I'm really not certain what all that stuff that's not in LaTeX is, but consider this:

If $A = \{a_{ij} \}$ then you know that 1) $A^T = \{ a_{ji} \}$
and you also know that 2) $(AB)^T = B^T A^T$.

Now if $B = \{ b_{jk} \}$ (where I've used the index "j" again since I know that j will iterate B in precisely the same manner as A for AB to make sense) you can write the matrix AB as $AB = \displaystyle \left\{ a_{ij} b_{jk} \right\}$. Now let's say, that without being too rigorous, you were to apply the operations from 1) and 2) to this sum, what would you get?

3. Oct 10, 2008

### qwerty5

[bkj][aij] ?

4. Oct 15, 2008

### max111

So I let A={a$$_{}ij$$} and B={b$$_{}ij$$}.

Then I know that A$$^{}T$$={a$$_{}ji$$} and B$$^{}T$$={a$$_{}ji$$}.

A typical element of the product B$$^{}T$$A$$^{}T$$={b$$_{}ji$$a$$_{}ji$$}.

However, B$$^{}T$$A$$^{}T$$={$$\Sigma$$$$^{}n$$$$_{}k=1$$b$$_{}jk$$a$$_{}kj$$}.

Is this correct?

5. Oct 15, 2008

### max111

So I let A={a$$_{}ij$$} and B={b$$_{}ij$$}.

Then I know that A$$^{}T$$={a$$_{}ji$$} and B$$^{}T$$={a$$_{}ji$$}.

A typical element of the product B$$^{}T$$A$$^{}T$$={b$$_{}ji$$a$$_{}ji$$}.

The sum would therefore be: B$$^{}T$$A$$^{}T$$={$$\Sigma$$b$$_{}jk$$a$$_{}ki$$} from k=1 to n.

Is this correct?

6. Oct 15, 2008

### qwerty5

Max, I think we are both right.

What did the professor get?