We present a new method of determining the anisotropy of the gap function inlayered high-Tc superconductors. Careful inelastic neutron scatteringmeasurements at low temperature of the phonon dispersion curves in the (100)direction in La_(1.85)Sr_(.15)CuO_4 would determine whether the gap ispredominately s-wave or d-wave. We also propose an experiment to determine thegap at each point on a quasi-two-dimensional Fermi surface.
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