Hi I am working on a project these days where I try to estimate the damping and spring stiffness. The way the physical model is described is by a single mass kelvin model. Therefore the mathematical model is described with the following homogenous ODE: mx’’ + cx’ + kx = 0 The problem is that I need to have a relationship between the damping and the spring stiffness. The k and c should also not be constants. They have to be non-linear. So the question is if there are some good theories which can help me describe such non-linearity for c and k?