“…A PDS of cardinality γ pr (G) we call a γ pr (G)-set. Paired-domination was introduced by Haynes and Slater [14], [15] as a model for assigning backups to guards for security purposes, and is studied, for example, in [2], [3], [4], [6], [9], [13], [16], [17], [18], [19], [20] and elsewhere.…”