Memoirs of the American Mathematical Society 2002; 83 pp; softcover Volume: 160 ISBN10: 0821829882 ISBN13: 9780821829882 List Price: US$59 Individual Members: US$35.40 Institutional Members: US$47.20 Order Code: MEMO/160/760
 Intersection homology theory provides a way to obtain generalized Poincaré duality, as well as a signature and characteristic classes, for singular spaces. For this to work, one has had to assume however that the space satisfies the socalled Witt condition. We extend this approach to constructing invariants to spaces more general than Witt spaces. We present an algebraic framework for extending generalized Poincaré} duality and intersection homology to singular spaces \(X\) not necessarily Witt. The initial step in this program is to define the category \(SD(X)\) of complexes of sheaves suitable for studying intersection homology type invariants on nonWitt spaces. The objects in this category can be shown to be the closest possible selfdual "approximation" to intersection homology sheaves. It is therefore desirable to understand the structure of such selfdual sheaves and to isolate the minimal data necessary to construct them. As the main tool in this analysis we introduce the notion of a Lagrangian structure (related to the familiar notion of Lagrangian submodules for \((1)^k\)Hermitian forms, as in surgery theory). We demonstrate that every complex in \(SD(X)\) has naturally associated Lagrangian structures and conversely, that Lagrangian structures serve as the natural building blocks for objects in \(SD(X).\) Our main result asserts that there is in fact an equivalence of categories between \(SD(X)\) and a twisted product of categories of Lagrangian structures. This may be viewed as a Postnikov system for \(SD(X)\) whose fibers are categories of Lagrangian structures. The question arises as to which varieties possess Lagrangian structures. To begin to answer that, we define the modelclass of varieties with an ordered resolution and use block bundles to describe the geometry of such spaces. Our main result concerning these is that they have associated preferred Lagrangian structures, and hence selfdual generalized intersection homology sheaves. Readership Graduate students and research mathematicians interested in geometry and topology. Table of Contents  Introduction
 The algebraic framework
 Ordered resolutions
 The cobordism group \(\Omega_\ast^{SD}\)
 Lagrangian structures and ordered resolutions
 Appendix A. On signs
 Bibliography
