On the Uniqueness Theorem for Pseudo-Additive Entropies

We discuss the idea that the Tsallis-type (q-additive) entropic chain rule allows for a wider class of entropic functionals than previously thought. In particular, we point out that the ensuing entropy solutions (e.g., Tsallis entropy) can be determined uniquely only when one fixes the prescription for handling conditional entropies. Our point is substantiated with the Darótzy's mapping theorem and DeFinetti-Kolmogorov theorem for escort distributions and illustrated with a number of examples. Connection with Landsberg's classification of non-extensive thermodynamical systems is also briefly discussed.