ε = 12λ1-1 + λ2-1 + λ3-1 - 3

This equation is the expression for the strain energy density (ε) in a material under a state of stress, where λ1, λ2, and λ3 are the principal strains. The equation is derived from the strain energy function, which relates the strain energy in a material to the deformation it undergoes. The equation is commonly used in the study of mechanics of materials and solid mechanics.