If c = 3, N=7 this is what it does
Assume I = [0.5,1,-3,0,1,15,1], j=2, k=3. We start at c=4 and sum to N-1=6, so three terms.
The entry in the matrix corresponding to row k, column j (entry (2,3)) is computed as
I_(4-3)*I(4-2)+I_(5-3)*I(5-2)+I_(6-3)*I(6-2) = I_1*I_2 + I_2*I_3 + I_3*I_4 = 0.5*1+1*(-3)+(-3)*0 = -2.5
