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I discuss four unpublished letters of Deligne (one on Hodge theory, two on Euler–Poincaré characteristics and ramification of ℓ[12pt]{minimal} {amsmath} {wasysym} {amsfonts} {amssymb} {amsbsy} {mathrsfs} {upgreek} {}{-69pt} {document}{document}-adic sheaves, one on generalized divisors), and sketch some of the developments they generated.
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Luc Illusie (2015) studied this question.
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