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Abstract
High temperature superconductors and ferroelectric oxides, and especially perovskites, have a variety of features in common which are suggestive to assume basically similar mechanisms to be responsible for their occurrence. While for ferroelectrics it has been shown that the local nonlinear electron-phonon interaction at the oxygen ion lattice site drives the structural instability, the origin of high temperature superconductivity is still not known. In the following it is argued that also high temperature superconductivity can be understood by phonon mediated electron-(hole) pairing, with similar electron-phonon interactions as occurring in ferroelectrics. Most importantly, these interactions are pseudoharmonically renormalized in ferroelectrics, while in superconductors an extension of conventional BCS theory is obtained. These two different treatments of the same unconventional electron-phonon interaction induce the important conclusion that ferroelectricity and superconductivity are mutually exclusive phenomena.