Sheaf (mathematics)

In mathematics, a sheaf F on a given topological space X gives a set or richer structure F(U) for each open set U of X. The structures F(U) are compatible with the operations of restricting the open set to smaller subsets and gluing smaller open sets to obtain a bigger one. A presheaf is similar to a sheaf, but it may not be possible to glue. Sheaves, it turns out, enable one to discuss in a refined way what is a local property, as applied to a function.

History

The first origins of sheaf theory are hard to pin down — they may be co-extensive with the idea of analytic continuation. It took about 15 years for a recognisable, free-standing theory of sheaves to emerge from the foundational work on cohomology.

Related Topics:
Analytic continuation - Cohomology

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• 1936 Eduard Čech introduces the nerve construction, for associating a simplicial complex to an open covering.
• 1938 Hassler Whitney gives a 'modern' definition of cohomology, summarizing the work since J. W. Alexander and Kolmogorov first defined cochains.
• 1943 Norman Steenrod publishes on homology with local coefficients.
• 1945 Jean Leray publishes work carried out as a POW, motivated by proving fixed point theorems for application to PDE theory; it is the start of sheaf theory and spectral sequences.
• 1947 Henri Cartan reproves the de Rham theorem by sheaf methods, in correspondence with André Weil. Leray gives a sheaf definition in his courses via closed sets (the later carapaces).
• 1948 The Cartan seminar writes up sheaf theory for the first time.
• 1950 The 'second edition' sheaf theory from the Cartan seminar: the sheaf space (éspace étalé) definition is used, with stalkwise structure. Supports are introduced, and cohomology with supports. Continuous mappings give rise to spectral sequences. At the same time Kiyoshi Oka introduces an idea (adjacent to that) of a sheaf of ideals, in several complex variables.
• 1951 The Cartan seminar proves the Theorems A and B based on Oka's work.
• 1953 The finiteness theorem for coherent sheaves in the analytic theory is proved by Cartan and Jean-Pierre Serre, as is Serre duality.
• 1954 Serre's paper Faisceaux algébriques cohérents (published 1955) introduces sheaves into algebraic geometry. These ideas are immediately exploited by Hirzebruch, who writes a major 1956 book on topological methods.
• 1955 Alexander Grothendieck in lectures in Kansas defines abelian category and presheaf, and by using injective resolutions allows direct use of sheaf cohomology on all topological spaces, as derived functors.
• 1956 Oscar Zariski's report Algebraic sheaf theory, Scientific report on the Second summer Institute : Several complex variables , Part III., Bull. Amer. math. Soc., t. 62, 1956, p. 117-141.
• 1957 Grothendieck's Tohoku paper rewrites homological algebra; he proves Grothendieck duality (i.e., Serre duality for possibly singular algebraic varieties).
• 1958 Godement's book on sheaf theory is published. At around this time Mikio Sato proposes his hyperfunctions, which will turn out to have sheaf-theoretic nature.
• 1957 onwards: Grothendieck extends sheaf theory in line with the needs of algebraic geometry, introducing: schemes and general sheaves on them, local cohomology, the derived category (with Verdier), and the Grothendieck topology. There emerges also his influential schematic idea of 'six operations' in homological algebra.

At this point sheaves had become a mainstream part of mathematics, with use by no means restricted to algebraic topology. It was later discovered that the logic in categories of sheaves is intuitionistic logic (this observation is now often referred to as Kripke-Joyal semantics, but probably should be attributed to a number of authors). This shows that some of the facets of sheaf theory can also be traced back as far as Leibniz.

Related Topics:
Algebraic topology - Intuitionistic logic - Kripke-Joyal semantics - Leibniz

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