  In this paper, we introduce the concept of mixed (G, S)-monotone mappings and
prove coupled coincidence and coupled common fixed point theorems for such
mappings satisfying a nonlinear contraction involving altering distance
functions. Presented theorems extend, improve and generalize the very recent
results of Harjani, L\'opez and Sadarangani [J. Harjani, B. L\'opez and K.
Sadarangani, Fixed point theorems for mixed monotone operators and applications
to integral equations, Nonlinear Analysis (2010), doi:10.1016/j.na.2010.10.047]
and other existing results in the literature. Some applications to periodic
boundary value problems are also considered.
