function Strain = isotropicinv(Stress, lambda, mu)
  tra = -lambda/2/mu/(2*lambda+2*mu) * (Stress(1) + Stress(2));
  Strain(1) = tra + Stress(1)/2/mu;
  Strain(2) = tra + Stress(2)/2/mu;
  Strain(3) = Stress(3)/2/mu;
  