# X_tanspose*x vs. x*x_transpose

1. Apr 17, 2009

### flufypancakes

x is a vector - (x1,x2, ..., xn)_transpose (i.e. a column vector). so when we have x_transpose*x we have x1^2+ x2^2+...+xn^2 = norm(x)^2.

right...

now what's x*x_transpose, i.e. column times the row? is it an n-by-n matrix?

actually now that i'm finished typing it i'm pretty sure it is, but i'll still post it to be sure..

2. Apr 17, 2009

### Cantab Morgan

What does your asterisk mean? Sometimes an asterisk means the complex conjugate of a transpose, but I'm not sure that's how you mean it here.

3. Apr 17, 2009

### flufypancakes

* means "times" as in 2*2=4

4. Apr 18, 2009

### matt grime

The definition of matrix multiplication is that when you compose an i by j matrix with a j by k matrix you get an i by k matrix.