The purpose of this paper is to present an example of a C 1 (in the Fréchet sense) discrete dynamical system in a infinite-dimensional separable Hilbert space for which the origin is an exponentially asymptotically stable fixed point, but such that its derivative at the origin has spectral radius larger than unity, and this means that the origin is unstable in the sense of Lyapunov for the linearized system. The possible existence or not of an example of this kind has been an open question until now, to our knowledge. The construction is based on a classical example in Operator Theory due to Kakutani.MSC: 37C75; 47A10, 43D20, 35B35.Keywords: Lyapunov stability of fixed points, linearization in infinite dimensions, dynamical systems.
Introduction and description of Kakutani's exampleLet X be a real Banach space and T : X → X a map such that T (0) = 0 and T is differentiable in the Fréchet sense at x = 0. If we call M = DT (0), then we can write T (x) = Mx + N(x) and the nonlinear part N(x) satisfies N(x) / x → 0 as x → 0. It is well-known and very easy to prove that if the spectral radius of M is less than one then the origin is exponentially assimptotically stable, in the sense of Lyapunov, as a fixed point †Instituto de Ciências Matemáticas e