“…At a linear level, this problem has been treated by several authors, mainly motivated by the dynamics of the so-called Aubry-Mather-Mañé sets (see, e.g. [16,29], [33, section 2.6], [4]), and problems from celestial mechanics ( [23,26]). Maslov-index arguments have also been used in this setting, see, e.g., [45].…”