“…Liu, Agarwal and Kang [14] , Liu and Kang [15] , Liu and Kim [16] , Liu and Ume [17] , Liu, Ume and Kang [18] , and Liu, Xu, Ume and Kang [19] also obtained analogous results for certain classes of functional equations different from the functional equation (1.2). By using common fixed point theorems, Huang, Lee and Kang [10] , Liu [11,12] and Zhang [20] investigated the existence and uniqueness of solutions for a few kinds of systems of functional equations arising in dynamic programming.…”