Robin-type conditions in plane strain

Let S ⊂ ℝ² be a domain bounded by a simple closed C²-contour ∂S of unit outward normal vector ν, and suppose that S is occupied by a homogeneous and isotropic material with Lamé constants λ and μ. If x = (x₁, x₂)^T is a generic point in ℝ², then the equilibrium equations of plane strain in the absence of body forces can be written in the form (1)A(∂x)u(x)=A(∂/∂x1,∂/∂x2)u(x)=0,    x∈S,https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780367812027/499b2676-17a6-4805-9fd8-78c8baf0b813/content/eq273.tif"/>