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Time: Thursday, May 03, 2018, 18h-20h

Colin McLarty ( - Personal website)

Grothendieck considered SGA 4 and 5 one single seminar, and his most profound finished mathematics (ReS pp. 373 and 274). The part of it which he put into print himself is titled 'Théorie des topos et cohomologie étale des schémas' and he complains often in ReS about later contempt for the idea of topos. Yet his own account of 'building houses' explains how he designed étale cohomology to be usable without knowing how it was conceived. The talk will survey the actually uses of toposes in SGA 4--7 and Grothendieck's grounds for saying 'we advise the reader nonetheless to assimilate the language of topos which provides an extremely convenient unifying principle.'


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