  Using a simple model for the frequency dependent scattering rate, we evaluate
the in-plane optical integral for cuprate superconductors in the normal and
superconducting states. In the overdoped region, this integral is conserved. In
the optimal and underdoped region, though, the optical integrals differ,
implying a lowering of the in-plane kinetic energy in the superconducting
state. This sum rule violation, due to the difference of the non Fermi liquid
normal state and the superconducting Fermi liquid state, has a magnitude
comparable to recent experimental results.
