The XV International Workshop on Condensed Matter Theories was held at the beautiful seaside resort of Mar del Plata, Argentina, during the first week of July, 1991. The first meeting of this workshop took place at the Instituto de Ffsica Teorica, Sao Paulo, Brazil, in 1977, as the first Panamerican Workshop on Condensed Matter Theories. Its purpose was to bring together scientists from the Western countries, to work on many different topics related to the manifold aspects of condensed matter theories. The Workshop was so successful in facilitating exchanges of ideas and techniques pertaining to different areas of scientific endeavour that it quickly transformed itself into a broadly based, interdisciplinary forum for the informal discussion of the interrelation and mutual connections that naturally arise between the diverse disciplines encompassed under the common name condensed matter theories. From the green-house effect to neural networks, all theoretical efforts are interwined in a very complex fashion. The next five workshops were held at Trieste, Italy (1978); Buenos Aires, Argentina (1979); Caracas, Venezuela (1980); Mexico City, Mexico (1981); and St. Louis, Missouri (1982). At the last meeting, in view of the truly international dimension reached by these gatherings, it was decided to substitute the word "International" for "Panamerican", and hold the next meeting, appropriately enough, in Europe.

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Formal Methods: A Reaction Theory and the Relation Between Widths and Energy Shifts; F.B. Malik. The Control of Chaos; B.A. Huberman. Classical and Quantum Fluids: Compressible RayleighBenard Convection in a Hard Disks System; D. Risso, P. Cordero. Positron Annihilation as a Probe of Localized States in Fluids; B.N. Miller. Electronic Systems, Atoms, and Molecules: Lower Bound Aspects of Fermion Density Functionals; J.K. Percus. Selfconsistent Semiclassical Mean Field; M. Casas, et al. High Tc Superconductivity: Search of Superconductivity in Metal Clusters; M. Barranco, et al. Stochastic Processes: Reactant Segregation; H.S. Wio, et al. Nuclear and Particle Physics: A Direct Approach to the Tamm-Dancoff Approximation; R.C. Bochicchio, H. Grinberg. Overview Talk: Some Considerations on the Greenhouse Effect and Related Problems; V.M. Canuto. 30 additional articles. Index.