1. Two points, P₃ and P₄, that are collinear with two given points, P₁ and P₂, such that P₃ lies in the line segment P₁P₂ while P₄ lies outside it, and, if x₁, x₂, x₃, and x₄ are the abscissas of the points, (x₃ - x₁)/ (x₃ - x₂) = - (x₄ - x₁)/(x₄ - x₂).

2. A pair of harmonic functions, u and v, such that u + iv is an analytic function, or, equivalently, u and v satisfy the Cauchy-Riemann equations.