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Non-differentiability of the effective potential and the replica symmetry breaking in the random energy model

The effective potential for the two-replica system of the random energy model is exactly derived. It is an analytic function of the magnetizations of two replicas, $φ^1$ and $φ^2$ in the high-temperature phase. In the low-temperature phase, where the replica symmetry breaking takes place, the effective potential becomes non-analytic when $φ^1=φ^2$. The non-analyticity is considered as a consequence of the condensation of the Boltzmann measure, which is a typical property of a glass phase.