W algebras are nonlinear generalizations of Lie algebras that arise in the context of two-dimensional conformal field theories when one explores higher-spin extensions of the Virasoro algebra. They provide the underlying symmetry algebra of certain string generalizations which allow the extended world sheet gravity. This book presents such gravity theories, concentrating on the algebra of physical operators determined from an analysis of the corresponding BRST cohomology. It develops the representation theory of W algebras needed to extend the standard techniques which were so successful in treating linear algebras. For certain strings corresponding to WN gravity we show that the operator cohomology has a natural geometric model. This result suggests new directions for the study of W geometry.
In particular, we calculate the physical spectrum of the noncritical D=4 W_3 as a problem in BRST cohomology. The corresponding operator cohomology forms a BV-algebra. We model this BV-algebra on that of the polyderivations of a commutative ring on six variables with a quadratic constraint, or, equivalently, on the BV-algebra of (polynomial) polyvector fields on the base affine space of SL(3,C). In this book we attempt to present a complete summary of the progress made in these studies.
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