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edited Apr 18 2010 at 9:43

Kevin Lin
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In accordance with the suggestion of Yemon Choi, I am going to suggest some further delineation of the approaches to "Non-commutative Algebraic GeometryGeometry". Because I know very little about "Non-commutative Differential GeometryGeometry", or what often falls under the heading "àa à la Connes". This will be completely underrepresented in this summary. For that I trust Yemon's summary to be satisfactory. (edit by YC: BB is kind to say this, but my attempted summary is woefully incomplete and may be inaccurate in details; I would encourage anyone reading to investigate further, keeping in mind that the NCG philosophy and toolkit in analysis did not originate and does not end with Connes.)

Additionally, much useful discussion took place at Kevin Lin's comment question (as Ilya stated above)in his answer).

Following the philosophy of Grothendieck: "to do geometry, one needs only the category of quasi-coherent sheaves on the would-be space"...space" (edit by KL: Where does this quote come from?)

In the famous dissertation of Gabriel, he introduced the injective spectrum of an abelian category, and then reconstructed the commutative noetherian scheme, which is a starting point of noncommutative algebraic geometry. Later, A. Rosenberg introduced the left spectrum of a noncommutative ring as an analogue of the prime spectrum in commutative algebraic geometry, and generalized it to any abelian category. He used one of the spectra to reconstruct any quasi-separated (not necessarily quasi-compact), commutative scheme.(Gabriel-Rosenberg scheme. (Gabriel-Rosenberg reconstruction theorem.)

Their approach is related to $A^{\infty}$ A_{\infty}$ algebras and homological mirror symmetry. References that might help are the papers of Soibelman. Also, I think this is related to the question here. (Note: (Note: I know hardly anything beyond that this approach exists. If you know more, feel free to edit this answer! Thanks for your understanding!)

(Some comments by KL: I am not sure whether it is appropriate to include Kontsevich-Soibelman's deformation theory here. This kind of deformation theory is a very general thing, which intersects some of the "noncommutative algebraic geometry" described here, but I think that it is neither a subset nor a superset thereof. In any case, I've asked some questions related to this on MO in the past, see this and this.

However, there is the approach of noncommutative geometry via categories, as elucidated in, for instance, Katzarkov-Kontsevich-Pantev. Here the idea is to think of a category as a category of sheaves on a (hypothetical) non-commutative space. The basic "non-commutative spaces" that we should have in mind are the "Spec" of a (not necessarily commutative) associative algebra, or dg associative algebra, or A-infinity algebra. Such a "space" is an "affine non-commutative scheme". The appropriate category is then the category of modules over such an algebra. Definitively commutative spaces, for instance quasi-projective schemes, are affine non-commutative schemes in this sense: It is a theorem of van den Bergh and Bondal that the derived category of quasicoherent sheaves on a quasi-projective scheme is equivalent to a category of modules over a dg algebra. (I should note that in my world everything is over the complex field; I have no idea what happens over more general fields.) Lots of other categories are or should be affine non-commutative in this sense: Matrix factorization categories (see in particular Dyckerhoff), and probably various kinds of Fukaya categories are conjectured to be so as well.

Anyway I have no idea how this kind of "noncommutative algebraic geometry" interacts with the other kinds explained here, and would really like to hear about it if anybody knows.)

Approach of Artin,Van Artin, Van den Berg school

Artin and Schelter gave a regularity condition on algebras to serve as the algebras of functions on non-commutative schemes. They arise from abstract triples which are understood for commutative algebraic geometry.(Again geometry. (Again edits are welcome!)

Here is a nice report on Interactions between noncommutative algebra and algebraic geometry. There are several people who are very active in this field: Michel Van den Berg,James Zhang,Paul smith,Toby stafford,IBerg, James Zhang, Paul Smith, Toby Stafford, I. Gordon, A.Yekutieli A. Yekutieli. There is also a very nice page of Paul Smith: noncommutative geometry and noncommutative algebra, where you can find almost all the people who are currently working in the noncommutative world.

Olav Laudal has approached NCAG using NC-deformation theory. He also applies his method to invariant theory and moduli theory.(Please theory. (Please edit!)

tidied up some of the English

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edited Feb 7 2010 at 10:09

Yemon Choi
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In accordance with the suggestion of Yemon Choi, I am going to suggest some further delineation of the approaches to Non-commutative Algebraic Geometry. Because I know very little about Non-commutative Differential Geometry, or what often falls under the heading "A àa la Connes". This will be completely underrepresented in this summary. For that I trust Yemon's summary to be satisfactory. (edit by YC: BB is kind to say this, but my attempted summary is woefully incomplete and may be inaccurate in details; I would encourage anyone reading to investigate further, keeping in mind that the NCG philosophy and toolkit in analysis did not originate and does not end with Connes.)

Additionally, much useful discussion took place at Kevin Lin's comment(As comment (as Ilya stated above).

Following the philosophy of Grothendieck:"to Grothendieck: "to do geometry, one need needs only the category of quasi coherent quasi-coherent sheaves on would be space" the would-be space"...

In the famous dissertation of Gabriel Des catégories abéliennes, he introduced the injective spectrum of an abelian category, and then reconstruct reconstructed the commutative noetherian scheme, which is a starting point of noncommutative algebraic geometry. Later, A.Rosenberg A. Rosenberg introduced the left spectrum of a noncommutative ring as an analogue of the prime spectrum in commutative algebraic geometry, and generalized it to any abelian category, he . He used one of the spectrum spectra to reconstruct any quasi-separated (not necessarily quasi compact)commutative quasi-compact), commutative scheme.(Gabriel-Rosenberg reconstruction theoremtheorem.)Additionally

In addition, A. Rosenberg has described the NC-Localization(first NC-localization (first observed also by Gabriel) which has been used by he him and Kontsevich to build NC analogs of more classical spaces(like spaces (like the NC Grassmannian) and more generally,Noncommutative stack. Additionallygenerally, Anoncommutative stacks. Rosenberg has also developed the Homological Algebra homological algebra associated to these 'spaces'. Applications of this approach include representation theory(D-module theory (D-module theory in particular), quantum algebra, and physics.

References in this area are best found through the MPIM Preprint Series, and a large collection are is linked here. Additionally, a book is being written by Rosenberg and Kontsevich furthering the work of their previous paper. Some applications of these methods are used here, here, here, and here. The first two are focusing on representation theory, the second two on non-commutative localization.

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edited Feb 7 2010 at 6:09

Shizhuo Zhang
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Following the philosophy of Grothendieck:"to do geometry, one needs need only the category of quasi coherent sheaves on the would be space" In his the famous dissertation of GabrielDes catégories abéliennes,Gabriel he introduced the injective spectrum of an abelian category and then reconstructed reconstruct the commutative noetherian scheme ; the which is a starting point of noncommutative algebraic geometry. Later, A. Rosenberg A.Rosenberg introduced the left spectrum of a noncommutative ring as an analogue of the prime spectrum in commutative algebraic geometry and generalized it to any abelian category. Using these spectrums, he is able to used one of the spectrum reconstruct any quasi-separated(not quasi-separated (not necessarily quasi-compact)commutative scheme. This is known as the Gabriel-Rosenberg quasi compact)commutative scheme.(Gabriel-Rosenberg reconstruction theorem. ) Additionally, A. Rosenberg has described the NC-Localization(first observed also by Gabriel) which has been used by he and Kontsevich to build NC analogs of more classical spaces(like the NC Grassmannian) and more generally, the Noncommutative generally,Noncommutative stack. Additionally, A. Rosenberg has developed the Homological Algebra associated to these 'spaces'. Applications of this approach include representation theory(D-module theory in particular), quantum algebra, and physics.

Artin-Schelter Regularity Condition

Approach of Artin,Van den Berg school

Here is a nice report on Interactions between noncommutative algebra and algebraic geometry.There are several people who are very active in this field: Michel Van den Berg,James Zhang,Paul smith,Toby stafford,I. Gordon, A.Yekutieli There is also a very nice page of Paul Smithnoncommutative geometry and noncommutative algebra, you can find almost all the people who are currently in noncommutative world.

Serre's FAC is the starting point of noncommutative projective geometry. But the real framework is built by Artin and James Zhang in their famous paperNoncommutative Projective scheme.

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edited Feb 7 2010 at 5:51

B. Bischof
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edited Feb 7 2010 at 5:14

Shizhuo Zhang
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[made Community Wiki]

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answered Feb 6 2010 at 23:18

B. Bischof
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Also note that much of what I know about these approaches comes from two sources:

The paper by Mahanta
My advisor A. Rosenberg.

Additionally, much useful discussion took place atKevin Lin's comment(As Ilya stated above).

I think a better break down for the NCAG side would be:

A. Rosenberg/Gabriel/Kontsevich approach

Building the Non-commutative Spectrum using Grothendieck categories and considering the category of Quasi-Coherent Sheaves on a 'Space'. Additionally, A. Rosenberg has described the NC-Localization which has been used by he and Kontsevich to build NC analogs of more classical spaces(like the NC Grassmannian). Additionally, A. Rosenberg has developed the Homological Algebra associated to these 'spaces'. Applications of this approach include representation theory, quantum algebra, and physics.

References in this area are best found through MPIM Preprint Series and a large collection are linked here. Additionally, a book is being written by Rosenberg and Kontsevich furthering the work of their previous paper. Some applications of these methods are used here, here, here, and here. The first two are focusing on representation theory, the second two on non-commutative localization.

Kontsevich/Soibelman approach

They might refer to their approach as "formal deformation theory", and quoting directly from their book

The subject of deformation theory can be defined as the "study of moduli spaces of structures...The subject of this book is formal deformation theory. This means $\mathcal{M}$ will be a formal space(e.g. a formal scheme), and a typical category $\mathcal{W}$ will be the category of affine schemes..."

Their approach is related to $A^{\infty}$ algebras and homological mirror symmetry. References that might help are the papers of Soibelman. Also, I think this is related to the question here.(Note: I know hardly anything beyond that this approach exists. If you know more, feel free to edit this answer! Thanks for your understanding!)

Lieven Le Bruyn's approach

As I know nearly nothing about this approach and the author is a visitor to this site himself, I wouldn't dare attempt to summarize this work.

As mentioned in a comment, his website contains a plethora of links related to non-commutative geometry. I recommend you check it out yourself.

Artin-Schelter Regularity Condition

References: This paper introduced the need for the regularity condition and showed the usefulness. Again I defer to Mahanta for details.

Non-commutative Deformation Theory by Laudal

Olav Laudal has approached NCAG using NC-deformation theory. He also applies his method to invariant theory and moduli theory.(Please edit!)

References are on his page here and this paper seems to be a introductory article.

Apologies

Without a doubt, I have made several errors, given bias, offended the authors, and embarrassed myself in this post. Please don't hold this against me, just edit/comment on this post until it is satisfactory. As it was said before, the nlab article on noncommutative geometry is great, you should defer to it rather than this post.

Thanks!