\Delta \left[ {x_i \left( n \right) - x_i \left( {n - \tau _i } \right)} \right] +

p_i \left( n \right)f_i \left( {x_1 \left( {n - \tau _{i1} } \right), \cdot \cdot \cdot ,x_m \left( {n - \tau _{im} } \right)} \right) = 0

and

\Delta ^2 y_i \left( {n - 1} \right) + \frac{{p_i (n)}} {{\tau _i }}f_i (y_1 (n), \cdot \cdot \cdot ,y_m (n)) = 0

where p_i (n): \mathbb{Z}^ + \to \mathbb{R}^ + , \tau _i \in \mathbb{Z}^ + ,\tau _{ij} \in \mathbb{Z} for i,j = 1, \cdot \cdot \cdot, m, f_i (u_1, \cdot \cdot \cdot, u_m) is nondecreasing in u_i for i = 1, \cdot \cdot \cdot, m and satisfies the following conditions

{ f_i (u_1, \cdot \cdot \cdot, u_m) \ge 0 if u_i \ge 0 for i = 1, \cdot \cdot \cdot, m,

f_i (u_1, \cdot \cdot \cdot, u_m) \le 0 if u_i \le 0 for i = 1, \cdot \cdot \cdot, m.