In accordance with the suggestion of Yemon Choi, I am going to suggest some further delineation of the approaches to "Non-commutative Algebraic Geometry". I know very little about "Non-commutative Differential Geometry", or what often falls under the heading "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.)
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 at Kevin Lin's question (as Ilya stated in his answer).
I think a better break down for the NCAG side would be:
A. Rosenberg/Gabriel/Kontsevich approach
Following the philosophy of Grothendieck: "to do geometry, one needs only the category of quasi-coherent sheaves on the would-be 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 reconstruction theorem.)
In addition, Rosenberg has described the NC-localization (first observed also by Gabriel) which has been used by him and Kontsevich to build NC analogs of more classical spaces (like the NC Grassmannian) and more generally, noncommutative stacks. Rosenberg has also developed the homological algebra associated to these 'spaces'. Applications of this approach include representation 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 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.
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!)
(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][22] and [this][23].
However, there is the approach of noncommutative geometry via categories, as elucidated in, for instance, [Katzarkov-Kontsevich-Pantev][24]. 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][25] (see in particular [Dyckerhoff][26]), 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.)
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.
Approach of 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 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, I. Gordon, 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.
References: [This][16] paper introduced the need for the regularity condition and showed the usefulness. Again I defer to [Mahanta][17] for details.
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 paper [Noncommutative Projective scheme][18].
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][19] and [this][20] 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][21] article on noncommutative geometry is great, you should defer to it rather than this post.
Thanks!
[16]: https://books.google.com/books?hl=en&lr=&id=_BnSoQSKnNUC&oi=fnd&pg=PA33&dq=%252522Artin%252522+%252522Some+algebras+associated+to+automorphisms+of+elliptic+curves%252522+&ots=hRXnP7udMW&sig=t77CnWnsYPHhuonQQffrSXedyj0#v=onepage&q="Artin" "Some algebras associated to automorphisms of elliptic curves"&f=false
[17]: https://arxiv.org/abs/math/0501166
[18]: https://web.archive.org/web/20121023193142/http://www.ingentaconnect.com/content/ap/ai/1994/00000109/00000002/art01087
[19]: https://web.archive.org/web/20181103123848/http://folk.uio.no:80/arnfinnl/
[20]: https://web.archive.org/web/20080425144650/http://folk.uio.no/arnfinnl/Noncom.alg.geom.pdf
[21]: https://ncatlab.org/nlab/show/noncommutative%20geometry
[22]: What is a deformation of a category?
[23]: Deformation theory and differential graded Lie algebras
[24]: https://arxiv.org/abs/0806.0107
[25]: Matrix factorizations and physics
[26]: https://arxiv.org/abs/0904.4713
Dear Kevin,
This is more or less an amplification of Tyler's comment. You shouldn't take it too seriously, since I am certainly talking outside my area of expertise, but maybe it will be helpful.
My understanding is that homotopy theorists are extremely (perhaps primarily) interested in torsion phenomena. (After all,
homotopy groups are often non-trivial but finite.) TMF, for example, involves quite subtle torsion phenomena. Coupled with Tyler's remark that homotopy theorists have no fear of $E_{\infty}$ rings, and so are (a) happy to identify them
with dg-algebras in char. zero, and (b) don't feel any psychological need to fall back on
the crutch of dg-algebras, this makes me suspect that your assumption (1) is likely to be wrong. (I share your motivation (2), but this is a psychological weakness of algebraists that
homotopy theorists seem to have overcome!)
In particular, one of Lurie's achievements is (I believe) constructing equivariant versions of TMF,
which (as I understand it) involves (among other things) studying deformations of $p$-divisible groups of derived elliptic curves. It seems hard to do this kind of thing
without having a theory that can cope with torsion phenomena.
Also, when Lurie thinks about elliptic cohomology, he surely includes under this umbrella TMF and its associated torsion phenomena. (So your (3) may not include all the aspects
of elliptic cohomology that Lurie's theory is aimed at encompassing.)
Best Answer
Charles,
a couple of reasons why a complex algebraic geometer (certainly someone who is interested in moduli spaces of vector bundles, as your profile tells me) might at least keep an open verdict on the stuff NC-algebraic geometers (NCAGers from now on) are trying to do.
in recent years ,a lot of progress has been made towards understanding moduli spaces of semi-stable representations of 'formally smooth' algebras (think 'smooth in the NC-world). in particular when it comes to their etale local structure and their rationality. for example, there is this book, by someone.
this may not seem terribly relevant to you until you realize that some of the more interesting moduli spaces in algebraic geometry are among those studied. for example, the moduli space of semi-stable rank n bundles of degree 0 over a curve of genus g is the moduli space of representations of a certain dimension vector over a specific formally smooth algebra, as Aidan Schofield showed. he also applied this to rationality results about these spaces.
likewise, the moduli space of semi-stable rank n vectorbundles on the projective plane with Chern classes c1=0 and c2=n is birational to that of semi-simple n-dimensional representations of the free algebra in two variables. the corresponding rationality problem has been studied by NCAG-ers (aka 'ringtheorists' at the time) since the early 70ties (work by S.A. Amitsur, Claudio Procesi and Ed Formanek). by their results, we NCAGers, knew that the method of 'proof' by Maruyama of their stable rationality in the mid 80ties, couldn't possibly work.
it's rather ironic that the best rationality results on these moduli spaces (of bundles over the projective plane) are not due to AGers but to NCAGers : Procesi for n=2, Formanek for n=3 and 4 and Bessenrodt and some guy for n=5 and 7. together with a result by Aidan Schofield these results show that this moduli space is stably rational for all divisors n of 420.
further, what a crepant resolution of a quotient singularity is to you, is to NCAGers the moduli space of certain representations of a nice noncommutative algebra over the singularity.
likewise, when you AGers mumble 'Deligne-Mumford stack', we NCAGers say 'ah! a noncommutative algebra'.