We assume now that we have an estimate for the unknown functions u11, u12, u21 and u22. We will call this the zeroth value and label it as u(0)11, u(0)12, u(0)21 and u(0)22. We can then set up an iterative scheme where the next solution is defined in terms of the previous one as u(1)11=14(b1−u(0)12−u(0)21)u(1)12=14(b2−u(0)11−u(0)22)u(1)21=14(b3−u(0)11−u(0)22)u(1)22=14(b4−u(0)12−u(0)21), where we have defined the vector b=[u01+u10−˜ρ11u13+u02−˜ρ12u31+u20−˜ρ21u32+u23−˜ρ22].