Talk:Descent (mathematics)

comments

I find the second part of the following sentence (starting with ": more") in the intro quite unclear: "The reason for abstraction here is, at a fundamental level, that passage to a quotient space is not very well-behaved in topology: more accurately, it is a tribute to the efforts to use category theory to get round the alleged 'brutality' of imposing equivalence relations within geometric categories."--MarSch 09:51, 14 October 2005 (UTC)

I have expanded the introduction, but I suppose you may not like it much better yet. Charles Matthews 10:40, 14 October 2005 (UTC)

In the section on history I find comment on "the representable functor question in algebraic geometry in general". I thought representable functors formed a language for describing many constructions (and associated problems of existence), not a single open question looming large. Have I been missing something? 128.135.60.45 03:54, 8 August 2007 (UTC)

Requested move

The following discussion is an archived discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. Editors desiring to contest the closing decision should consider a move review. No further edits should be made to this section.

The result of the move request was: moved. —Darkwind (talk) 01:36, 23 April 2013 (UTC)

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Descent (category theory) → Descent (mathematics) – Obviously, the language of category theory is essential to the descent theory. But that doesn't make the topic a part of category theory. Thus, it's better to use a more neutral article title. Taku (talk) 12:39, 5 April 2013 (UTC)

Agreed, WP:PRECISION makes this uncontroversial, with no currently competing article to consider. ᛭ LokiClock (talk) 15:49, 5 April 2013 (UTC)

Disagree. Descent in category theory is still rather specific, although it is an attempt to generalise various arguments. It is not the same as Proof by infinite descent, which is often simply called "descent" in the context of Diophantine equations, for example, in the proof of the Mordell–Weil_theorem (see for example Elliptic_curve#The_structure_of_rational_points). Deltahedron (talk) 11:56, 7 April 2013 (UTC)

This is a shorthand. It would not be introduced without the full name in a textbook. A hatnote would suffice. e.g., In the elliptic curves article you cited, there is a source that refers simply to descent in the title. The first mention of descent in that thesis reads "infinite descent." ᛭ LokiClock (talk) 17:24, 7 April 2013 (UTC)

The above discussion is preserved as an archive of a requested move. Please do not modify it. Subsequent comments should be made in a new section on this talk page or in a move review. No further edits should be made to this section.

Introduction

The description given in the introduction sounds rather different from what I understand by descent in this context. Chapter 8 of Pedicchio & Tholen express it as follows. A problem on a base object B is solved for an extension E and then the solution is taken down to B by some kind of "projection" p:E→B. Examples would include a sheaf over a topological space, or a module over a ring.

The Grothendieck description is then presented as a fibration F:D→C so that the category of objects over B in C is given by the fibre F^-1(B) and descent theory gives a description, hopefully effective, of that fibre in terms of the fibre over E. Deltahedron (talk) 12:02, 7 April 2013 (UTC)

Yes, especially the second para reads very weirdly. Why "equivalence relation"? for instance (related to cocycle conditions?). What I remember is similar to what you described. The very basic case would be like: we have a field extension $L/K$. One proves a theorem for objects (e.g., varieties) over L then tries to "descend" the theorem to K; whence, the term. More categorically, we have $f:\operatorname {Spec} L\to \operatorname {Spec} K$ (i.e., projection) and consider the pullback $f^{*}$ that is a functor between categories of sheaves. Usually this is not simple since one has to consider the flat topology instead of Zariski topology. In any case, this is what I remember. -- Taku (talk) 21:38, 7 April 2013 (UTC)

Assessment comment

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*Close to start class. A reference would help. Geometry guy 00:28, 3 June 2007 (UTC) Rerated to MID. A critical concept and tool in modern algebraic geometry (for moduli spaces, as an example). Moreover a valuable organsing principle for many other topics. The present article has not yet reached a level that would make this clear, I'm afraid. Could be classified in (algebraic) geometry or topology as well. Stca74 17:24, 14 June 2007 (UTC)

Last edited at 04:16, 23 April 2013 (UTC). Substituted at 01:59, 5 May 2016 (UTC)