  In a system with strong local repulsive interactions it should be more
difficult to add an electron than to extract one. We make this idea precise by
deriving various exact sum rules for the one-particle spectral function
independent of the details of the Hamiltonian describing the system and of the
nature of the ground state. We extend these results using a variational ansatz
for the superconducting ground state and low lying excitations. Our results
shed light on the striking asymmetry in the tunneling spectra of high Tc
superconductors and should also be useful in estimating the local doping
variations in inhomogeneous materials.
