MATLAB: Norm square

How to implement equation:

CRE=||Pu||^2

P is a matrix

Is Pu a vector resulting from multiplying a vector by a matrix

P = randn(2,2);
u = randn(2,1);
norm(P*u,2)

No, it is not the average.

Assuming you intend the 2-norm, the norm of a matrix is the largest singular value.

Use an anonymous function:

 nm.a = cellfun(@(m) norm(m, 'fro'), a)