“…This latter approach, inspired by [40], turned out to be flexible enough to also allow for the treatment of the space-time case [13,24], and, together with some of the computations carried out in [28,29], is probably the method that displays the largest number of analogy with ours. On this regard, it should be mentioned that level set methods, combined with Bochner technique and integral identities, have recently found some applications to the study of static metrics in general relativity [3,6], even when the function under consideration is not necessarily harmonic [10][11][12].…”