Caleb Eckhardt
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Maximal ideals of ultraproducts of full matrix algebras
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16 votes

I think Nik Weaver is right that the ideal mentioned is the unique maximal ideal. This simultaneously answers both questions (since the quotient is clearly infinite dimensional). Let $\tau$ be the ...

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Reference request: Brown Ozawa and strong completely positive approximation property?
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10 votes

I am not sure if it is in Brown and Ozawa, but it is in Pisier's recent book "Tensor Products of C*-algebras and Operator Spaces" as Corollary 10.16. It may also be in his earlier Operator ...

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Von Neumann Algebra isomorphism extension
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10 votes

No. This will almost never be true (subalgebras of the compacts are the only cases I can think of where it could work). The easiest example is probably $C[0,1].$ It has an injective homomorphism to $...

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Continuity of the product map
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10 votes

I think the answer is: It's always continuous if $A$ is subhomogeneous and never continuous otherwise(min or max). First notice that if the product map is continuous for min, then it's continuous for ...

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What are the possible eigenvalues of these matrices?
9 votes

This is a partial answer that shows an obstruction to certain eigenvalue sequences. First, I claim that if $M$ is rank one then it isn't similar to something of the stated form. Take a matrix $M$ ...

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Can a non-commutative C*-algebra be a minimal operator space?
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9 votes

I don't think so. If A is a noncommutative C*-algebra with the min operator space structure then it's double dual $A^{**}$ will also have the min operator space structure (this fact can be found in ...

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Norm estimation of identity plus two non-commuting self-adjoint operators
8 votes

(Unless I messed up the arithmetic, here's a counterexample.) Let $B=\begin{bmatrix} 1 & 1 \\ 1 & 1 \end{bmatrix}$ and $A=\begin{bmatrix} 1 & 0 \\ 0 & R \end{bmatrix}$ where $R$ is a ...

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Uniform Roe algebra of virtually abelian group is type I C*-algebra?
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7 votes

It is not Type I in general. Probably it is not Type I whenever $G$ is infinite. Here is an argument when $G=\mathbb{Z}.$ Consider the projections in $\ell^\infty(\mathbb{Z})$ defined by ...

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Do irreducible characters form a closed set?
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7 votes

No. Here's an argument that shows the answer is no for non-abelian, torsion free, finitely generated two-step nilpotent groups (this argument could easily be pushed to nilpotent groups of steps ...

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C$^*$-algebras isomorphic after tensoring with $M_n(\mathbb C)$
7 votes

This is not an answer, but too long for a comment and just an idea for an approach. Kirchberg algebras (unital, purely infinite, separable, nuclear) in the UCT class are classified by a triple $(G_0, ...

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Does this C*-algebra embed into a simple nuclear C*-algebra?
7 votes

Your algebra is Type I and residually finite dimensional (RFD) Therefore it's unitization is also Type I and RFD. Let's call $B$ it's unitization. Since $B$ is Type I it satisfies the UCT and ...

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Is the bitranspose continuous for the $\sigma$-strong topology?
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6 votes

I think the transpose map on the compacts gives a counterexample. Let $K(H)$ be the compacts on a separable infinite dimensional Hilbert space with orthonormal basis $\{ e_n \}.$ Let $T:K(H)\...

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hereditary C*-subalgebra of a non-elementary simple C*-algebra
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6 votes

This is true in the separable case (and more generally) and a consequence of Larry Brown's stable isomorphism theorem (1977 Pacific Journal of Math). A special case of his theorem states: If $A$ is ...

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Maps which are both completely positive and positive
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6 votes

No. I use facts about Schur multipliers which can be found, for example in Paulsen's monograph on completely bounded maps. Basically you are searching for a positive semidefinite matrix with positive ...

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Extensions of sub-homogeneous $C^*$-algebras are subhomogeneous
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6 votes

Yes. Since $C^{**}\cong A^{**}\oplus B^{**}$, uniform bounds on the dimension of the irreducible representations of $A$ and $B$ impose a uniform bound on the dimension of the irreducible ...

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maximal tensor product of simple $C^*$algebras is non-simple
6 votes

An explicit example to question 1 is given by $A=B=C^*_r(G)$ where $G$ is the free group on two generators. Takesaki produced the first systematic study of nuclearity (then called Property (T)) in ...

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Representation is a compact perturbation of a direct sum of one-dimensional representations
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4 votes

In practice, there are several non-obvious corollaries of Voiculescu's theorem that are collectively called "by Voiculescu's Theorem." In this case it is a consequence of Voiculescu's theorem (the one ...

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Folium in GNS construction and von Neumann algebras
4 votes

It will be rare to find a C*-algebra which has one folium for all states. Most likely you need to require that it's dual space is norm separable (so you can do it for say subalgebras of compact ...

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Is the extension of full free group c^* algebra a group?
4 votes

Ext is a group for the full case. Kirchberg proved in Lemma 3.3 of [Commutants of unitaries in UHF algebras and functorial properties of exactness. J. Reine Angew. Math. 452 (1994), 39–77] that the ...

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Uniqueness of conditional expectations defined on $C(\mathbb{T})$
3 votes

The conditional expectation it is not unique. Basically the different positive definite functions on the quotient group $\mathbb{Z}/k\mathbb{Z}$ will produce other conditional expectations. Here is ...

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Real rank 0 implies stable rank 1 on $C^\ast$-algebras?
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3 votes

There are "cheap" examples. If a C*-algebra $A$ has stable rank 1 then it is stably finite. On the other hand every simple, unital purely infinite C*-algebra has real rank 0. So every simple, ...

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Finitely generated $K_0$ of $C^*$-algebras
3 votes

No. To obtain a counterexample, you just need a C*-algebra $A$ with finitely generated K-theory and a quotient $A/I$ of $A$ which does not have finitely generated K-theory and the quotient ...

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unital embedding into the coner $C^*$-algebra
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3 votes

Using your notation, we have diag$(x',...,x')\in M_n\otimes M_m(A).$ Then $M_n$ is isomorphic to the algebra (let's call it $B$) generated by $e_{ij}\otimes x'$ where $e_{ij}$ are matrix units for $...

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