Combining ideas of Ihara-Serre-Tate, Lang [5] proved the following natural result. If a (complex, irreducible) plane curve C ⊂ A contains infinitely many points with both coordinates roots of unity, then C is the zero locus of an equation of the form xy = ζ, where a, b ∈ Z and ζ is a root of unity. In other words, if F ∈ C[x, y] is an irreducible polynomial for which there exist infinitely many...