Constructing projective modules

Abstract

We discuss elements of a social history of the theory of projective modules over commutative rings. We attempt to study the question: how did the theory of projective modules become one of "mainstream" focus in mathematics? To do this, we begin in what one might call the pre-history of projective modules, describing the mathematical culture into which the notion of projective module was released. These recollections involve four pieces: (a) analyzing aspects of the theory of fiber bundles, as it impinges on algebraic geometry, (b) understanding the rise of homological techniques in algebraic topology, (c) describing the influence of category-theoretic ideas in topology and algebra and (d) revisiting the story of the percolation of sheaf-theoretic ideas through algebraic geometry. We will then argue that it was this unique confluence of mathematical events that allowed projective modules to emerge as objects of central mathematical importance. More precisely, we will first argue that, in the context of social currents of the time, projective modules initially were isolated as objects of purely technical convenience reflecting the aesthetic sensibilities of the creators of the fledgling theory of homological algebra. Only later did they transcend this limited role to become objects of "mainstream importance" due to influence from the theory of algebraic fiber bundles and the theory of sheaves. Along the way, we aim to show how strong personal ties emanating from the Bourbaki movement and its connections in mathematical centers including Paris, Princeton and Chicago were essential to the entrance, propagation and mainstream mathematical acceptance of the theory.