On Sat, Mar 15, 2014 at 4:00 PM, Charles R Harris <charlesr.harris@gmail.com> wrote:

These days they are usually written as v*w.T, i.e., the outer product of two vectors and are a fairly common occurrence in matrix expressions. For instance, covariance matrices  are defined as E(v * v.T)

With the current numpy, we can do

>>> x = arange(1, 5)

>>> x[:,None].dot(x[None,:])

array([[ 1,  2,  3,  4],

       [ 2,  4,  6,  8],

       [ 3,  6,  9, 12],

       [ 4,  8, 12, 16]])

I assume once @ becomes available, we will have

>>> x[:,None] @ x[None,:]

array([[ 1,  2,  3,  4],

       [ 2,  4,  6,  8],

       [ 3,  6,  9, 12],

       [ 4,  8, 12, 16]])