[Numpy-discussion] help using np.einsum for stacked matrix multiplication

[Dave Hirschfeld]

Andrew Nelson writes:

> 
> Dear list,I have a 4D array, A, that has the shape (NX, NY, 2, 2).  I 
wish to perform matrix multiplication of the 'NY' 2x2 matrices, resulting 
in the matrix B.  B would have shape (NX, 2, 2).  I believe that np.einsum 
would be up to the task, but I'm not quite sure of the subscripts I would 
need to achieve this.
> 
> Can anyone help, please?
> 
> cheers,
> Andrew.

Sorry, I'm a little unclear on what is supposed to be multiplied with 
what. You've got (NX x NY) 2x2 matricies, how do you end up with NX 2x2 
matricies?

Perhaps if you show the code using loops with `np.dot` it will be clearer 
how to translate to using `np.einsum`

-Dave