Pesin's identity provides a profound connection between entropy hKS (statistical mechanics) and the Lyapunov exponent λ (chaos theory). It is well known that many systems exhibit sub-exponential separation of nearby trajectories and then λ = 0. In many cases such systems are non-ergodic and do not obey usual statistical mechanics. Here we investigate the non-ergodic phase of the PomeauManneville map where separation of nearby trajectories follows δxt = δx0e λαt α with 0 < α < 1. The limit distribution of λα is the inverse Lévy function. The average λα is related to the infinite invariant density, and most importantly to entropy. Our work gives a generalized Pesin's identity valid for systems with an infinite invariant density.PACS numbers: 05.90.+m, 05.45.Ac, 74.40.+k Chaotic systems are characterized by exponential separation of nearby trajectories, which is quantified by a positive Lyapunov exponent λ [1]. Such a behavior leads to the need for statistical approaches since chaos implies our inability to predict the long time limit of a system in a deterministic fashion. A profound relation between chaos and statistical mechanics is given by Pesin's identity [1]. It states that the Kolmogorov-Sinai entropy h KS is equal to the Lyapunov exponent λ for a closed ergodic 1d systems (to the sum of positive Lyapunov exponents for d > 1). At the same time, it is well known that many systems such as Hamiltonian models with mixed phase space [2], systems with long range forces [3], certain billiards [4] and one-dimensional hard-particle gas [5] have a Lyapunov exponent equal zero. While for complex systems it may be extremely difficult to determine whether the Lyapunov exponent is zero or small, due to numerical inaccuracies, it turns out that most fundamental text book examples of chaos theory may have a zero Lyapunov exponent. Prominent examples for such weakly chaotic systems are the logistic map at the edge of chaos (Feigenbaum's point) [6] and the Pomeau-Manneville map which is used to model intermittency (originally in turbulence) [7]. If the Lyapunov exponent is zero, i.e. separation of trajectories is sub-exponential, we have a strong indication that the usual Boltzmann-Gibbs statistical mechanics is not valid. Indeed it was found that certain systems with zero Lyapunov exponents break ergodicity [8,9]. Classical entropy theory is also not applicable in this case [6,7], particularly the entropy and average algorithmic complexity grow non linearly in time [7], while for a system with a positive Lyapunov exponent they increase linearly in time. Still the situation is not hopeless from the point of view of statistical mechanics and one may consider distributions of time average observables [8,9,10,11,12].Connection between possible generalizations of usual statistical mechanics and weakly chaotic systems have attracted much attention recently. In particular, a generalized Pesin's identity for the logistic map at the edge of chaos was investigated using Tsallis statistics [6,13]. A critical discussion of this appr...