# Matrix Theory Transpose

#### macro90

##### New Member
I'm studying General Linear Model, so I have to go over Matrix Theory. I don't get what is said below:

Shouldn't $$yy^T$$ be $$([n \times 1][1 \times n])$$? and therefore be undefined because we can't do matrix multiplication here?? I thought matrix multiplication was not commutative.

#### quark

Shouldn't $$yy^T$$ be $$([n \times 1][1 \times n])$$? and therefore be undefined because we can't do matrix multiplication here?? I thought matrix multiplication was not commutative.
$$yy^T$$ = $$([n \times 1][1 \times n])$$, a $$n \times n$$ matrix.
Let's try a simple case where $$y^T$$ = (1,1).
$$yy^T$$ = $$\left( \begin{array}{cc}1 & 1\\1 & 1\\\end{array} \right)$$
$$y^Ty$$=2