From the analyticity properties of the equation governing infinitesimal perturbations, it is shown that all stability properties of spatially extended 1D systems can be derived from a single function that we call entropy potential since it gives directly the Kolmogorov-Sinai entropy density. Such a function allows determining also Lyapunov spectra in reference frames where time-like and space-like axes point in general directions in the space-time plane. The existence of an entropy potential implies that the integrated density of positive exponents is independent of the reference frame.

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