SYMMETRY AND NODES OF THE SUPERCONDUCTING GAP

For pairing potentials which act only in the vicinity of the Fermi-sur face, a Fermi-surface harmonics expansion of the gap function is appro priate. After a general symmetry discussion, it is pointed-out that th e first extended s-wave occuring in the Fermi-surface harmonics expans ion of the gap function a) is closely related to the usual d-waves b) it can have a large number of nodes. This may reconcile the experiment s which see nodes in the gap and neutron-scattering experiments which cannot resolve the anistropy in the gap function.