Approximation algorithms for finding and partitioning unit-disk graphs into co-k-plexes

This article studies a degree-bounded generalization of independent sets called co-k-plexes. Constant factor approximation algorithms are developed for the maximum co-k-plex problem on unit-disk graphs. The related problem of minimum co-k-plex coloring that generalizes classical vertex coloring is also studied in the context of unit-disk graphs. We extend several classical approximation results for independent sets in UDGs to co-k-plexes, and settle a recent conjecture on the approximability of co-k-plex coloring in UDGs. Key-words: unit-disk graph; independent set; graph coloring; co-k-plex; k-dependent set; defective coloring; t-improper coloring