All Questions
Tagged with operator-algebras or oa.operator-algebras
2,153 questions
2
votes
0
answers
105
views
Multiplicativity of the analytic index (or of kernel bundle)
What I want to ask is the multiplicativity of the analytic index of a family of Dirac operators.
In the single operator case the analytic index of elliptic operator is multiplicative. This is proved ...
- 1,173
3
votes
1
answer
170
views
Strong and weak equivalence of C$^∗$-extensions by compacts
Let $A$ be a $C^*$-algebra. An extension of $A$ by the compact operators $K$ is an embedding $\epsilon$ of $A$ into the Calkin algebra $B(H)/K$.
Two embeddings $\epsilon_1$ and $\epsilon_2$ are ...
- 273
1
vote
0
answers
120
views
Tor functor in the case of algebra of smooth functions
Let $A=C^{\infty}(\mathbb{S}^{1})$, let $B$ be the sub-algebra $C^{\infty}(0,1)$. Here we identify $\mathbb{S}^{1}$ by $\mathbb{R}/2\pi \mathbb{Z}$. I want to ask if there is a way I can decompose $A$ ...
- 6,249
4
votes
1
answer
775
views
Algebraically simple Banach algebras
There are plenty of semi-simple Banach algebras - this broad class includes C*-algebras and algebras of bounded operators on a given Banach space. On the other hand, it seems unlikely to me that there ...
- 113
3
votes
0
answers
168
views
Integral representation of the complex homogeneous polynomial $z_1\cdots z_n$
Consider the transform (see e.g., (5.1) in this paper):
\begin{equation*}
\Lambda_\mu(q)(z) := \int_{\Delta_n} q(\zeta)\,\Re\left(\frac{1+\langle\zeta,z\rangle}{1-\langle\zeta,z\rangle}\right)d\mu(\...
- 28.6k
7
votes
2
answers
790
views
Question about von Neumann algebra generated by a complete algebra of projections
Hi all, sorry if this is a dumb question, I don't know much about von Neumann algebras except the definition and a few relevant facts I've managed to prove by myself so I expect the answer will turn ...
- 391
5
votes
1
answer
494
views
When is a $*$-homomorphism between multiplier algebras strictly continuous?
(This question was posted on MSE here but didn't get any answers.)
The strict topology on the multiplier algebra M(A) of a C*-algebra A is that generated by the seminorms
$$ x\mapsto \|ax\|\quad x\...
- 1,336
5
votes
0
answers
151
views
Submodules of a Hilbert space with finite Jones index with respect to a von Neumann algebra
While studying some basic theory of Cartan subalgebras of von Neumann algebras I found the following fact that I couldn't prove:
Let $H$ be a Hilbert space, $A$ and $B$ trace von Neumann subalgebras ...
- 743
4
votes
2
answers
329
views
Existence of irreducible subspace
Let $H$ denote a Hilbert space and let $\cal A$ be a subalgebra of the algebra ${\cal B}(H)$ of all bounded operators on $H$ such that $\cal A$ consists of compact operators only and such that each ...
user1688
3
votes
1
answer
166
views
Sets $E$ in $\mathbb{Z}$ such that any $l^2$ function with support on $E$ comes from Fourier of a continuous function
Are there infinite sets $E\subset\mathbb{Z}$ such that any $f\in l^2(\mathbb{Z})$ with support on $E$ comes from the Fourier transform of a continuous function on $\mathbb{T}$ ? If yes, is there a ...
- 647
2
votes
1
answer
244
views
$R$ is a right multiplier and $R(a)b=a\overset{?}{\implies} A$ is unital
Let $A$ be a $C^*$-algebra, and $R:A\to A$ its right multilplier. Is it true that
$$
\exists b\in A\quad \forall a\in A \quad R(a)b=a\qquad
$$
implies $A$ is unital. I know this is true if A is a weak$...
- 1,697
8
votes
1
answer
420
views
What is the general form of the duality transform for the Fock space?
I am interested in properties of the symmetric Fock space, looked at via the associated Wiener space. It is well known that for a Hilbert space $k$, the symmetric Fock space $$\mathcal{F}(L^2(\mathbb{...
- 81
3
votes
0
answers
229
views
The convolution on a semisimple finite quantum groupoid
Let $\mathbb{A}$ be a finite dim. weak Hopf $C^*$-algebra (or semisimple finite quantum groupoid) and $\hat{\mathbb{A}}$ its dual.
Let the Fourier transform $\mathcal{F}: \mathbb{A} \to \hat{\mathbb{...
1
vote
0
answers
131
views
Ideal structure of group $C^*$-agebras [closed]
Let $G$ be a locally compact groups and $C_r^*(G)$ be a reduce group $C^*$-algebra.
$\ Question:$What is the ideal structure of reduce group $C_r^*(G)$?
- 399
4
votes
1
answer
874
views
equality in noncommutative Hölder inequality
Let $1\leq p,q,r\leq \infty$ such that $\frac{1}{r}=\frac{1}{p}+\frac{1}{q}$. Let $S_p$ denote the Schatten space. For any $x\in S_p$ and any $y\in S_q$ we have
$$
||xy||_{S_r} \leq ||x||_{S_p}||y||_{...
- 1,222
-1
votes
1
answer
132
views
How should $A^α$ be defined for real $α ∈ [0,∞)$ and $A\in M_n(\mathbb C)$? [closed]
Let $A\in M_n(\mathbb C)$ be arbitrary. I'm interested to know How should $A^{\alpha}$ be defined for real $\alpha\in [0,\infty)$? When $A$ is nonsingular, we can define $A^{\alpha}=\exp(\alpha \log(A)...
- 201
6
votes
1
answer
323
views
A fourier series related to spin Chern numbers almost commuting matrices
Let
$$
f(x)=\sin(x)\sqrt{1+\cos^{2}(x)+\cos^{4}(x)}.
$$
In my study of almost commuting unitary matrices, $U$ and $V$, I
have need for a bound like
$$
\left\Vert \tilde{f}(V)U-U\tilde{f}(V)\right\...
- 1,749
3
votes
1
answer
293
views
Is the square diagram of index and exponential maps in $K$-theory of $C^*$-algebras anti-commutative?
Assume we have a $3\times 3$ grid with rows and columns being short exact sequences of $C^*$-algebras.
This gives a grid of 6-term exact sequences: 3 "horizontal" sequences and 3 "vertical" sequences,...
- 1,284
5
votes
1
answer
152
views
when is an algebra map conjugate to a star algebra map
Take a (unital) algebra map $f:A\to B$ between two unital C* algebras - not necessarily star preserving. Under what circumstances is there a $b\in B$ so that $g(a)=b\ f(a)\ b^{-1}$ is a star algebra ...
- 1,143
4
votes
0
answers
135
views
References for a lemma about compact operators on a Hilbert module
I am looking for a reference for the following result:
If $A$ and $B$ are C* algebras, $H$ is a right Hilbert $A$-modules, $\phi :A \rightarrow B$ is a morphism, and assume that there is a map $\eta :...
- 42.4k
7
votes
0
answers
241
views
A "slice-map" type problem for symmetric tensors in the square of a nuclear C*-algebra
Throughout: let $\otimes$ denote the minimal (i.e. spatial) $\newcommand{\Cst}{{\rm C}^*}\Cst$-tensor product of two $\Cst$-algebras.
Let $B$ be a unital, nuclear $\Cst$-algebra and let $A\subset B$ ...
- 25.8k
18
votes
1
answer
1k
views
Commuting unitaries
Is the following true:
For every unit vectors $x_1,..., x_n$, $y_1,..., y_n$ in $\mathbb{C}^k$
there exist a Hilbert space $H$, unitary operators $U_1,...,U_n$ and $V_1,...,V_n$ in $B(H)$ and unit ...
- 4,705
2
votes
0
answers
79
views
Algebraic machinery for boundary conditions: may spectral data be "lifted" via the Toeplitz extension?
Let $\tilde{\mathcal H}$ be a Hilbert space, and let $L(\tilde{\mathcal H})$ denote the corresponding space of linear operators. By fixing a basis, we can, via Fourier transform, identify an important ...
- 605
7
votes
1
answer
811
views
Actions orbit equivalent to profinite ones
Let $G$ be a countable discrete residually finite group.
Is there a way to characterise the actions of $G$ that are orbit-equivalent to profinite ones?
Ozawa and Popa introduced the concept of ...
1
vote
0
answers
118
views
Finding the infimum of the range of a certain non-negative function associated to a $ C^{*} $-algebra
Let $ A $ be a non-trivial $ C^{*} $-algebra and $ n \in \mathbb{N} $. Setting $ \mathcal{D} \stackrel{\text{df}}{=} A^{n} \setminus \{ (0_{A},\ldots,0_{A}) \} $, we can define a function $ f: \...
- 1,396
7
votes
1
answer
408
views
Is a C* completion of a nuclear Fréchet algebra a nuclear C* algebra?
I am sure that this is well known in the right places, but: Is the C* completion of a star nuclear Fréchet algebra a nuclear C* algebra? (Suppose that the C* norm is continuous with respect to the ...
- 1,143
2
votes
1
answer
569
views
Functors with an epi-mono factorization property
This is a simple question about terminology and a request for any related references. Specifically, what would you call a functor $F:\mathbf{D}\rightarrow\mathbf{C}$ with the following property?
$(*)...
- 1,307
4
votes
2
answers
286
views
unitary reduction of $q$-normal matrices
The unitary reduction of normal matrices is a well-known fact: if $A\in M_n(\mathbb C)$ commutes with its Hermitian adjoint $A^*$, then there exists a unitary $U\in\mathbb U_n$ and a diagonal matrix $...
- 52.3k
4
votes
1
answer
175
views
CB-norm of mappings from a matrix space
The following result of Roger Smith is well known to operator algebraists:
$$\| \phi: E \rightarrow M_n\|_{cb} =\| \phi^{(n)} \otimes id_{M_n}: E \otimes_{min}M_n \rightarrow M_n \otimes M_n\|,$$
...
- 769
5
votes
1
answer
261
views
Jordan-Hölder theorem for planar algebras?
First recall the Jordan-Hölder theorem for groups:
Theorem (Jordan-Hölder): Let $G$ be a group, and let $$ G=G_1 \supset G_2 \supset \dots \supset G_r = \{ e \} $$ be a normal tower such that ...
3
votes
0
answers
57
views
Integration of Weyl operators multiplied by quasifree state over a symplectic space
I am reading the book "An invitation to the Algebra of Canonical Commutation Relations" by Denes Petz. It is freely available for download here. In Chapter 9, he defines the Lebesgue measure on a ...
5
votes
0
answers
141
views
Functoriality of $\mathsf{Cu}$
I have always been happy with the proof of the functoriality of the Cuntz semigroup $\mathsf{Cu}$ given in arXiv:0902.3381, where the isomorphism
$$\mathsf{Cu}(A)\cong W(A\otimes K)$$
is used, $A$ ...
- 417
10
votes
1
answer
1k
views
When are certain group C*-algebras exact?
This is somewhere between a "reference request" and "ask an expert", but I hope it is not too trivial or off-topic.
Anyway. There has been a lot of attention given to showing that for certain ...
- 25.8k
5
votes
2
answers
491
views
Is independence meaningful for commutative $C^*$-algebras?
I don't know very much about spectral theory so probably the answer to my question has a basic reference which I would appreciate.
Let's say I have two self-adjoint operators on a Hilbert space and ...
- 2,243
4
votes
1
answer
272
views
K-Theory of Algebra of Zeroth Order Pseudo differential operators
Any one knows a reference for computing K_0 of Algebra of zeroth order Pseudo's on a closed manifold in terms of explicit generators?
Thanx!
- 309
7
votes
1
answer
250
views
$K_0$ group of graph underlying an approximately finite (AF) C* algebra
Say we have an AF C* algebra $A$ described by some Bratteli diagram $E$. If $M_\infty (A)=\displaystyle{\lim_\rightarrow M_n(A)}$ and $P(A)$ are the projections in this algebra, we know that $K_0(A)^+=...
- 125
1
vote
0
answers
67
views
Inner representation and related group $C^*$-algebra
Let $G$ - discrete group. Consider $C^*$-algebra $C^*_\gamma(G)\subset B(\ell^2(G))$ which is generated by operators $T_g:\delta_x \mapsto \delta_{gxg^{-1}}$ where $g\in G$. Are there some good ...
7
votes
2
answers
587
views
Conditional expectations onto masas in type III factors
There is always a (unique)normal condition expectation onto a masa in a type II_1 factor. When does a masa in a type III factor admit a normal conditional expectation? (If we drop normality, ...
- 184
2
votes
0
answers
412
views
Two Definitions of Non-commutative $L^p$ space
Throughout, let $(\mathcal{M},\tau)$ be a von Neumann algebra $\mathcal{M}$, acting on a Hilbert space $H$, with normal semifinite faithful trace $\tau$.
In the survey article by Pisier and Xu, the ...
- 75
2
votes
1
answer
265
views
What is the multiplicative unitary for SU_q(2) (or other quantum groups)?
Consider a (von Neumann algebraic) locally compact quantum group $(M, \Delta, \phi, \psi)$ where the von Neumann algebra $M$ is realized as operators on the Hilbert space $H$. There is a ...
- 280
1
vote
0
answers
68
views
square root of contractions in Baer *-rings
Let $A$ be a unital Baer*-ring. We say that $a$ is a contraction if $aa^*\leq1$ and $a^*a\leq1$.
Q1) Assume $a$ is a contraction. Has the positive element $1-aa^*$ any square root?
(if yes, seems $1-...
- 4,058
2
votes
2
answers
422
views
non-Identity operator on a separable Hilbert space
Suppose $\mathcal{H}$ is a separable Hilbert space over $\mathbb{C}$ (countable dimensions) with inner product $\langle,\rangle$. Let $A$ be a bounded linear operator on $\mathcal{H}$, i.e, in $B(\...
- 103
5
votes
1
answer
508
views
Projections which are not completely bounded
There are 'canonical' examples of maps on operator spaces which are not completely bounded. Nevertheless, I couldn't produce any examples of bounded projections on relatively easy to understand ...
- 181
7
votes
2
answers
743
views
Would a supersymmetric theory of von Neumann algebras be useful?
While looking over the first chapter of
1) Quantum Fields and Strings: A Course For Mathematicians (P. Deligne, P. Etingof, D.S. Freed, L. Jeffrey, D. Kazhdan, J. Morgan, D.R. Morrison and E. Witten, ...
- 7,057
2
votes
0
answers
89
views
On finite index infinite depth subfactors and reduction to depth 2
Let $(N \subset M)$ be a finite index irreducible subfactor (with $N$ and $M$, ${\rm II}_1$ factors).
Let $N \subset M \subset M_1 \subset M_2 \subset \cdots$ be the tower of basic constructions.
...
1
vote
1
answer
611
views
Commutant of a von Neumann algebra as the linear span of unitaries.
I'm reading chapter 4 of Gerard Murphy's C*-algebras book and am confused by a statement in his proof of theorem 4.1.10. In his proof, he says, "$A'$ is the linear span of its unitaries" (where $A'$ ...
2
votes
1
answer
335
views
"Generators" of one-parameter groups of isometries
Let $E$ be a Banach space, and let $(\sigma_t)$ be a strongly continuous one-parameter group on $E$: so for $t\in\mathbb R$, we have that $\sigma_t$ is a contraction on $E$, $\sigma_t \sigma_s=\sigma_{...
- 18.7k
7
votes
0
answers
602
views
Unique maximal ideal in group C*-algebras
Let $G$ be a discrete group. Let $C^*(G)$ denote the full group C*-algebra of $G.$ Let $\pi:C^*(G)\rightarrow \mathbb{C}$ be the *-homomorphism associated with the trivial representation of $G.$
...
- 2,689
3
votes
1
answer
339
views
Pullbacks of $C^*$-algebras
I am reading the paper of Pedersen: "Pullback and Pushout Constructions in C^*-Algebra Theory". I try to work out the arguments from Proposition $3.1$ of his paper (you can find this article in the ...
- 31
2
votes
1
answer
298
views
An upper bound for the maximal subgroups at fixed index?
Let us call a subgroup an injective homomorphism between groups.
I warn the reader that a subgroup designates here an inclusion $(H \subset G)$, not $H$ alone.
A subgroup $H \subset G$ is ...