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The theory that the change of the electronic kinetic energy in a directionperpendicular to the CuO-planes in high-temperature superconductors is asubstantial fraction of the condensation energy is examined. It is argued thatthe consequences of this theory based on a rigorous $c$-axis conductivity sumrule are consistent with recent optical and penetration depth measurements.Comment: 4 pages (RevTeX) and 2 eps figure

Frustrated Kinetic Energy, the Optical Sum Rule, and the Mechanism of Superconductivity