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Noncommutative geometry in the sense of Connes and beyond: noncommutative algebras viewed as functions on a noncommutative space.

60 votes
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What is the significance of non-commutative geometry in mathematics?

$\DeclareMathOperator\coker{coker}$I think I'm in a pretty good position to answer this question because I am a graduate student working in noncommutative geometry who entered the subject a little bit …
Paul Siegel's user avatar
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22 votes

Penrose tilings and noncommutative geometry

I don't have any magical references for you, nor do I understand the NCG point of view on the Penrose tiling all that well. I learned just enough about this to convince myself that I didn't need to l …
Paul Siegel's user avatar
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16 votes
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Reference: Learning noncommutative geometry and C^* algebras

First of all, let me mention that functional analysis plays a similar role in noncommutative geometry that commutative algebra plays in algebraic geometry, and it pays off to at least have a reference …
Paul Siegel's user avatar
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15 votes
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A survey for various $K$-homology theories and their relationship

I would say that there are really only two definitions of K-homology commonly used in the literature (apart from the naive definition via the Bott spectrum): "analytic K-homology" and "geometric K-hom …
Paul Siegel's user avatar
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11 votes
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C*-algebras, foliations and dynamical systems

A very good place to start is Connes' book "Noncommutative Geometry", available for free on his website. It's a huge book, but it's possible to skip around quite a bit to get what you need. To begin …
Paul Siegel's user avatar
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5 votes
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Comparing the definitions of $K$-theory and $K$-homology for $C^*$-algebras

KK-theory provides one way to eliminate the asymmetry in the definitions of K-theory and K-homology. A cycle in $KK(A, B)$ is a triple $(H, \rho, F)$ where $H$ is a (adjectives) Hilbert $B$-module, $ …
Paul Siegel's user avatar
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5 votes

The "right" $C^*$ algebraic proof of Bott Periodicity

Probably the argument that you're looking for is based on the "Dirac / Dual Dirac" method. The idea is to exploit the product structure in KK-theory as much as possible - this makes the proof functor …
Paul Siegel's user avatar
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4 votes
0 answers
166 views

Minimal Diffeomorphisms and Cohomology

Let $M$ be a compact manifold and let $D$ be a minimal diffeomorphism from $M$ to itself (meaning there are no nontrivial invariant subspaces). I believe it was Connes who proved that if the first co …
Paul Siegel's user avatar
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3 votes
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Examples of $C^*$-algebras in Noncommutative Geometry from A. Connes

Maybe it will help to see how the algebra $B$ is a special case of the general construction in $2.\alpha$. The compact manifold is the space $Y = \{a, b\}$ consisting of two points. The open cover o …
Paul Siegel's user avatar
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