Three functions of position, ψ₁, ψ₂, and ψ₃, 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 = -2∂²ψ₁/∂x₂∂x₃, σ₂₃ = ∂²ψ₂/∂x₁∂x₂ + ∂²ψ₃/∂x₁∂x₃, and cyclic permutations of these equations.