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Algebraic and topological K-theory, relations with topology, commutative algebra, and operator algebras
23
votes
Conceptual explanation for the relationship between Clifford algebras and KO
Here is one conceptual description of the relationship. (I wouldn't call it an explanation; I don't really know why it's true, except that it's because Bott periodicity is true.)
KO-theory is the fi …
21
votes
Accepted
Are the Stiefel-Whitney classes of a vector bundle the only obstructions to its being invert...
The answer is no. Let $G$ be a cyclic group of order $n$ not divisible by $2$, let $V$ be an irreducible $2$-dimensional representation of $G$, and consider the associated vector bundle $EG\times_G V …
15
votes
Brauer Groups and K-Theory
To see why $K$-theory should be Brauer graded, it may help to see how, for a superalgebra $A$, the associated $K$-group only depends on the category $\mathcal{M}_A$ of finite dimensional $Z/2$-graded …
14
votes
Accepted
BU with tensor product H-space structure
I'll write $U(X)=[X,BU_\otimes]$. So $U(X)\subset K(X)=[X,Z\times BU]$ is the multiplicatively closed subset corresponding to "virtual bundles of rank 1".
Likewise, I'll write $I(X)=[X,BU]\subset K(X …
13
votes
Torsion in K-theory versus torsion in cohomology
There are many spaces $X$ whose $K$ theory is trivial (isomorphic to that of a point), but whose ordinary cohomology is not. Famously, there is a 4-cell complex obtained by taking the homotopy cofibe …
13
votes
Commutativity in K-theory and cohomology
Perhaps the deeper story you want involves the notion of "$E_{\infty}$ product". The cup product in cohomology, and the sum (and for that matter, the product) in K-theory are commutative (and associa …
10
votes
How do you relate the number of independent vector fields on spheres and Bott Periodicity fo...
This is not really answering your question. But it's worth pointing out that periodicity of Clifford algebras (closely tied to Bott periodicity) already gives you the "periodicity" in the explicit lo …
9
votes
Are the real and complex Adams operations compatible under the inclusions $U(n) \rightarrow ...
I think the answer is yes, it commutes.
The identities $rc=2$, $\psi^k c=c\psi^k$, and additivity of these operations already implies that $\epsilon:= \psi^k r-r\psi^k$ satisfies $2\epsilon =0$ in $[ …
5
votes
Accepted
Short question relating to the proof of the Atiyah-Singer Index Theorem for families
The answer should probably go something like this:
Both $q_!$ and $ind$ are $K(A)$-module maps. Since $q_!$ is an isomorphism, to check that these are the same maps, it suffices to check they are th …