On sharp stability of the BCC quantizer
We prove sharp nonlinear stability estimates for the squared-distance quantization energy of the body-centered cubic lattice. The reference Hessian has optimal coercivity constant \(1/96\) relative to nearest-neighbor displacement differences, uniformly over all finite-index periods. We classify all equality modes and prove cubic cancellation on their span. This yields nonlinear stability in a neighborhood independent of the period, including volume-preserving strain. The loss in the sharp energy coefficient is at most quadratic for each fixed period and exactly linear when all periods are allowed. We also determine the transition between these regimes. Finally, we prove global optimality and uniqueness of BCC, up to rigid motions, among two-site configurations with rectangular period cells of fixed volume.