IntroductionWe shall give a direct arithmetic approach to the problem of stability of some partial differential equations, in particular the equation of diffusion (in one and two dimensions), the wave equation and the equation of the vibrating bar. We do not attempt here to indicate the scope of the method. The results obtained by the usual analytic approach1 are recovered, and the operator theoretical treatment is motivated, We make considerable use of properties of special matrices and simple facts about the characteristic roots