for p > 3 definition
for p > 3 means taking the derivative of Equation 5.19 with respect to the composition of rth component on the trays with indices larger than 3. Because there is no composition term with the tray indices larger than 3 in Equation 5.19, the elements of F' becomes zero for p = 4 . . . (NT + 2) and r = 1 . . . NC. This is mathematically represented by Equation 5.20.