Survey on Roman {2}-Domination

The notion of Roman {2}-domination was introduced in 2016 as a variant of Roman domination, a concept inspired by a defending strategy used by the emperor Constantine (272–337 AD) to protect the Roman Empire. Since then, a considerable number of papers on Roman {2}-domination and its variants have been published. In this paper, we survey published results on Roman {2}-domination as well as the main findings on Roman {2}-domination variants found in the literature. A list of open problems related to this notion and its variants are also given.