“…there exists k [0, 1) such that d(Tx, Ty) ≤ kd(x, y) for all x, y X with x ≼ y; 2. there exists x 0 X such that x 0 ≼ Tx 0 ; 3. if {x n } is a nondecreasing sequence in X such that x n x X as n ∞, then x n ≼ x for all n. Then T has a fixed point. Since then, several authors considered the problem of existence (and uniqueness) of a fixed point for contraction type operators on partially ordered metric spaces (see, for example, [2,3,5,[15][16][17]19,[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]). …”