Calculating energy using MATLAB

Question

Energy, which is as follows:

can be found based on the MATLAB documentation using:

stats = graycoprops(glcm, properties)

For example, I typed this:

>> a = [1 2; 3 4]

a =

     1     2
     3     4

>> stats = graycoprops(a, {'energy'})

stats =

    Energy: 0.3000

If we want to do this manually, how did we get the energy value shown above?

This is the part I didn't get clearly.

Answer 1

If I'm not wrong:

ans=sum(a(:).^2)/sum(a(:)).^2