Using general Blatz-Ko strain energy function to solve an unit compressible cube. W=0.5*mu*alpha*[(J1-3)+beta*(J3^(-2/beta)-1)] +0.5*mu*(1-alpha)*[(J2-3)+beta*(J3^(2/beta)-1)] where J1=I1, J2=I2/I3, J3=I3^0.5, beta=(1-2*nu)/nu, nu is Poisson's ratio.