A Constructive Proof of the Spherical Parisi Formula

The Parisi formula for the free energy is among the crown jewels in the theory of spin glasses. We present a simpler proof of the lower bound in the case of the spherical mean-field model. Our method follows the TAP approach developed recently in e.g. (Subag, 2018): we obtain an ultrametric tree of pure states, each with approximately the same free energy as the entire model, which are hierarchically arranged in accordance with the Parisi ansatz. We construct this tree ``layer by layer'' given the minimizer to Parisi's variational problem. On overlap intervals with full RSB, the tree is built by an optimization algorithm due to Subag. On overlap intervals with finite RSB, the tree is constructed by a new truncated second moment argument; a similar argument also characterizes the free energy of the resulting pure states. Notably we do not use the Aizenman--Sims--Starr scheme, and require interpolation bounds only up to the 1RSB level. Our methods also yield results for large deviations of the ground state, including the entire upper tail rate function for all 1RSB models without external field.