Periodicity theorems in (generalized) cohomology theories - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-18T14:15:40Z http://mathoverflow.net/feeds/question/32296 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/32296/periodicity-theorems-in-generalized-cohomology-theories Periodicity theorems in (generalized) cohomology theories Paul Siegel 2010-07-17T17:15:01Z 2010-07-17T22:57:12Z <p>It is well-known that topological K-theory is blessed with the Bott periodicity theorem, which specifies an isomorphism between $K^2(X)$ and $K^0(X)$ (where $K^n$ is defined from $K^0$ by taking suspensions). I am wondering if other generalized cohomology theories have their own periodicity theorems, and if there is a general framework for conceptualizing them. I am interested in any substantive answer to this question, but there are two specific avenues for generalization that I am particularly curious about.</p> <p>The first avenue begins with the Clifford algebra approach to Bott periodicity. This approach relates periodicity in K-theory to a certain natural periodicity present in the theory of complex Clifford algebras, and it generalizes the 8-fold periodicity of real K-theory (corresponding to an 8-fold periodicity in real Clifford algebras). Can one fruitfully generalize the notion of a Clifford algebra, associate to it a generalized cohomology theory, and analogously produce a periodicity theorem?</p> <p>The second avenue involves Cuntz's proof of Bott periodicity for C*-algebras (which in particular implies topological Bott periodicity by specializing to commutative C*-algebras). Cuntz proves Bott periodicity for any functor from the category of C*-algebras to the category of Abelian groups which is stable (i.e. insensitive to tensoring with the C*-algebra of compact operators on Hilbert space), half exact, and homotopy invariant. The proof uses topological properties of Toeplitz algebras in an essential way. Because of the generality of his approach, I am left wondering if the essential features of his argument can be translated into more general contexts.</p> <p>Any ideas are welcome!</p> http://mathoverflow.net/questions/32296/periodicity-theorems-in-generalized-cohomology-theories/32298#32298 Answer by norondion for Periodicity theorems in (generalized) cohomology theories norondion 2010-07-17T17:33:09Z 2010-07-17T17:33:09Z <p>Cohomology theories constructed by the Landweber exact functor theorem are periodic, and there is also a periodization of cohomology theories.</p> http://mathoverflow.net/questions/32296/periodicity-theorems-in-generalized-cohomology-theories/32314#32314 Answer by skupers for Periodicity theorems in (generalized) cohomology theories skupers 2010-07-17T20:59:50Z 2010-07-17T20:59:50Z <p>In the spirit of first approach, there is a conjecture for a Clifford-algebra type proof of the 576-fold periodicity of TMF. This is a generalized cohomology theory constructed by piecing together all the elliptic cohomology theories together in a suitable way. I heard about this conjecture from Andre Henriques, who is working on a geometric approach to TMF using conformal nets.</p> <p>The idea is that the free fermion conformal net (a introduction can be found in <a href="http://arxiv.org/abs/0912.5307" rel="nofollow">this article</a> by Bartels, Douglas and Henriques) is to TMF as the Clifford algebras are to K-theory. For a suitable sense of equivalence, i.e. some generalization of Morita equivalence, $Free(n)$ and $Free(n+576)$ should be equivalent. I believe people are still far from a proof, but the motivation comes from looking at orientations: a manifold is orientable for K-theory if the frame bundle (a principal $SO(n)$-bundle) lifts to a principal $Spin(n)$-bundle. The $Spin(n)$ groups can be defined as a group in the Clifford algebra. For TMF, a manifold is orientable if the frame bundle extends to a principal $String(n)$-bundle, which can be obtained from $Spin(n)$ by killing $\pi_3$, just like $Spin(n)$ is obtained from $SO(n)$ by killing $\pi_1$. There is a way to define $String(n)$ using the free fermion conformal nets.</p> <p>The only reference I know for these ideas is the following <a href="http://www.staff.science.uu.nl/~henri105/Grants/CNTMF-B2.pdf" rel="nofollow">summary</a>.</p> http://mathoverflow.net/questions/32296/periodicity-theorems-in-generalized-cohomology-theories/32323#32323 Answer by Dylan Wilson for Periodicity theorems in (generalized) cohomology theories Dylan Wilson 2010-07-17T22:57:12Z 2010-07-17T22:57:12Z <p>In this <a href="http://empg.maths.ed.ac.uk/Videos/Atiyah80/Hopkins.mov" rel="nofollow">video</a> of a lecture given at Atiyah's 80th Birthday Conference, Mike Hopkins gives a description of the solution to the Kervaire Invariant Problem which relies heavily on some periodicity in a cohomology theory created for this problem. He mentions Clifford algebras and Bott Periodicity at one point towards the end of the talk... I don't know a whole lot about this topic so I didn't understand too much, but maybe it will be helpful for you!</p>