“…Arguing by contradiction, we assume that V is not a subsolution of (3.1) at (x 0 , y 0 ) with x 0 > 0 and y 0 > 0. With a similar proof to Proposition 3.1 of [9], but extending the definitions to two variables, we first show that there exists ε > 0, h ∈ (0, min{x 0 /2, y 0 /2}), and a continuously differentiable function ψ : R 2 + → R such that ψ is a test function for the subsolution of (3.1) at (x 0 , y 0 ) and satisfies…”