Three functions of position, ϕ₁, ϕ₂, and ϕ₃, in terms of which the elements of the stress tensor σ of a body may be expressed, if the body is in equilibrium and is not subjected to body forces; the elements of the stress tensor are given by σ₁₁=∂²ϕ₂/∂x₃²+∂²ϕ₃/∂x₂², σ₂₃= - ∂²ϕ₁/∂x₂∂x₃, and cyclic permutations of these equations.