By Giraud topos I mean all the assumptions in Giraud's theorem, exept a small set of generators. What Grothendieck call "faux topos". See SGA4 Exposse IV Theoreme 1.2 (Giraud's theorem) and Example 2.8 (faux topos). best e.d. I guess I was wrong when I thought that "Giraud Topos" was established terminology in the cat-list. On 11/8/16 09:59, Daniil Frumin wrote:
What is actually a Giraud topos? I cannot find a reference for this on the internet.
On Mon, Nov 7, 2016 at 10:03 PM, Eduardo Julio Dubuc <edubuc@dm.uba.ar <mailto:edubuc@dm.uba.ar>> wrote:
Hi, in this posting I will use the terminology used by most people in the list.
There are Grothendieck, Giraud and Elementary (Lawvere-Tierney) topos.
Grothendieck are Giraud and Elementary, my question is:
Are Elementary Giraud topos which are not Grothendieck ?
Examples ?
greetings e.d.
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Dear Eduardo Unless you are using a different statement of the Giraud's theorem than the one I have in mind, they are I think considerably more often called $\infty$-pretopos (like in the Elephant) or infinitary pretopos (like in the nLab) to avoid any confusion with an infinity categorical notion. I don't think I have ever encountered a different terminology (but I do like 'Giraud topos'). Regarding the example you are looking for, unless I'm missing something, the example 2.8 in SGA that you mentioned (the category sets endowed with smooth action of a large group) is also an elementary topos: sub-object classifier, exponential and power object are constructed exactly in the case of an ordinary group action topos and only involve a small quotient of the large group. So it answer you question. Bests, Simon
By Giraud topos I mean all the assumptions in Giraud's theorem, exept a small set of generators. What Grothendieck call "faux topos".
See SGA4 Exposse IV Theoreme 1.2 (Giraud's theorem) and Example 2.8 (faux topos).
best e.d.
I guess I was wrong when I thought that "Giraud Topos" was established terminology in the cat-list.
[For admin and other information see: http://www.mta.ca/~cat-dist/ ]
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Eduardo Julio Dubuc -
henry@phare.normalesup.org