Stiff systems of ordinary differential equations are a very important special case of the systems taken up in Initial Value Problems. There is no universally accepted definition of stiffness. Some attempts to understand stiffness examine the behavior of fixed step size solutions of systems of linear ordinary differential equations with constant coefficients. The eigenvalues λi of the Jacobian matrix completely characterize the stability of the system in this case. They also determine the behavior of explicit numerical methods applied to the system.