“…Consider ψ : Σ −→ R 3 an indefinite improper affine sphere, that is, an immersion with constant affine normal ξ and Lorentzian affine metric h. Then, see [10,17], up to an equiaffine transformation, one has ξ = (0, 0, 1) and ψ can be locally seen as the graph of a solution f (x, y) of (1.1).…”