Lower Bound on the Energy Gap at Precise Locations on the Bi_2Sr_2CaCu_2O_8 Fermi Surface

Recent inelastic-neutron-scattering measurements of a low-energy phonon in Bi_2Sr_2CaCu_2O_8 provide enough information to place a lower bound on the energy gap magnitude at precise locations on the Fermi surface. Measurements of a few low-energy phonons should indicate whether the gap symmetry is d_{x^2-y^2}. Further measurements could map out the angle-resolved gap magnitude on the entire Fermi surface.