# Questions tagged [banach-algebras]

The banach-algebras tag has no usage guidance.

351
questions

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### positive functional on Banach *-algebra (with appro. identity) is continuous?

Theorem (N. Th.Varopoulos): Let $\mathcal{B}$ be a Banach *-algebra
with a bounded approximate identity. Then every positive functional $T$ on $\mathcal{B}$ is continuous.
I think this theorem is ...

1
vote

0
answers

78
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### An example of a non rigid Banach algebra

A Banach algebra $A$ is called a rigid Banach algebra if for every injective Banach algebra morphism $J:A\to A$ we have either $\overline{J(A)}$ is ismorphic to $A$ or it does not contain ...

3
votes

1
answer

194
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### Closed prime ideal in $C[0, 1]$

I know that maximal ideals of $C[0, 1]$ corresponds to singleton. Also, using Zorn's lemma one can construct a prime ideal in $C[0, 1]$ which is not maximal.
Is there any $\textbf{closed}$ prime ...

0
votes

1
answer

75
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### Bound for the product of Sobolev functions in $W^{s,1}$

I would like to bound the product of two functions $f$, $g$ in the space $W^{s,1}$.
$$ \lVert fg\rVert_{W^{s,1}}\leq c\lVert f \rVert \lVert g \rVert. $$
It seems reasonable to want to use Hölder's ...

1
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0
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45
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### About isometric Banach algebra isomorphisms and WAP functionals

Let $B$ be a Banach algebra and $A\subseteq B$ a subalgebra. It is known that there is a quotient map $\phi: B^*/\textrm{wap}(B)\to A^*/\textrm{wap}(A)$ that is also an $A$-bimodule map.
Let's say ...

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0
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86
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### Representations of the dual Banach algebra pair $(\ell_1,c_0)$

Let $\displaystyle E_p=(\bigoplus_{n\in\mathbb{N}} \ell^1_n)_{\ell^p}$ for some $1<p<\infty$ and $\ell^1 = \ell^1(\mathbb{N})$ be equipped with the convolution. Then, there exists an isometric &...

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102
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### A dual Banach algebra question

Let $\Gamma$ be an infinite discrete abelian group and $A=\ell^1(\Gamma)$ denote its group algebra.
Clearly, $A_*=c_0(\Gamma)$ is a predual of $\ell^1(\Gamma)$ for which $(A,A_*)$ is a dual Banach ...

1
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0
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81
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### About vector valued measure algebras

Let $G$ be a locally compact group and $A$ be a Banach algebra. By $L^1(G,A)$ and $M(G,A)$ we denote the $A$-valued group, and measure algebra.
Is $M(G,A)$ a Banach algebra (with convolution as the ...

1
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0
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61
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### Algebras sitting inside reproducing kernel Hilbert space other than multiplier algebra

Suppose $\mathcal{H}$ is a reproducing kernel Hilbert space. If the kernel is normalized then the multiplier algebra $\mathcal{M}$ is an algebra that is sitting inside $\mathcal{H}$. Is there any ...

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0
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56
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### Weierstrass subdomain of $\DeclareMathOperator\Spm{Spm}\Spm \mathbb{Q}_p$

I am trying to understand Weierstrass subdomains of $\Spm\DeclareMathOperator\QP{\mathbb{Q}_p}\QP$.
Recall that a Weierstrass algebra of an affinoid space $\Spm A$, where $A$ is a Banach algebra with ...

2
votes

0
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326
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### Weakly null sequences in projective tensor products

First, I'd like to record a question that may still be open. The snippet below is taken from DiestelPuglisi2009.
Second, let $E$ be a Banach space, $(u_n)$ be a weakly null sequence in the projective ...

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0
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62
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### Two questions about the vector-valued Lipschitz algebra

For a commutative Banach algebra $A$ and for any $0<\alpha<1$, let $\text{Lip}_\alpha(K,A)$ consist of all $A$-valued functions $f$ on a metric space $(K,\text d)$ with the property that $\rho_\...

3
votes

0
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283
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### Dunford-Pettis like properties for Banach spaces of operators

Let $E$ be a Banach space and $A\subseteq B(E)$ be a Banach subspace of operators on $E$.
Suppose $A$ satisfies the property (RCC) given below:
$$
\left.\begin{array}{l}
(x_n)\subseteq A \textrm{ ...

2
votes

0
answers

86
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### The group of quasi unitary elements of a (simple) Banach algebra

For a Banach algebra $A$ with invertible group $G(A)$ we define the following group:
$$QG(A)=\{u\in G(A)\mid \;\text{the mapping}\; a\mapsto u^{-1} a u \;\text{is an isometry}\}$$
What is an ...

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0
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136
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### Could we characterize elements in the second dual by the character space?

Let $A$ and $B$ be two semisimple commutative Banach algebras. Assume that $A\mathbin{\tilde\otimes} B$ is a Banach algebra obtained by completing $A\otimes B$ with respect to a cross norm not ...

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1
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302
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### Maximal ideals of the ring $\mathbb C \{T\}$

Consider the Banach $\mathbb C$-algebra
$$
\mathbb C \{T\} = \left\lbrace \sum_{i \geq 0} a_i T^i : \sum_{i \geq 0} |a_i| < \infty \right\rbrace
$$
With the norm given by $\| \sum a_i T^i\| = \sum |...

2
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0
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### Commutative Banach algebras with zero-dimensional maximal ideal space and disjoint supports of Gelfand transforms

Let $A$ be a commutative semi-simple unital Banach algebra and let $\Delta$ be the maximal ideal space of $A$. Denote by $\widehat{\cdot}\colon A\to C(\Delta)$ the Gelfand transform.
If $\Delta$ is ...

3
votes

0
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36
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### Norm under Gelfand map vs norm under left regular representation on $\ell^p$

Let $G$ be a discrete commutative group. Let $p \in [1,\infty)$ and consider the left regular representation $\lambda : \ell^1(G) \to \mathcal{B}(\ell^p(G))$; that is $\lambda(x)y := x*y$,
where
$$
(x*...

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0
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### Tracial linear functionals on an amenable Banach algebra

This post is related to an earlier question about Kazhdan property (T). The purpose of the snippet below is to briefly summarize the background for the question in this post.
Question: Does there ...

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152
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### Kazhdan's property (T) for Banach algebras?

A locally compact group $G$ has Kazhdan's property (T) if the trivial representation $1_G:G\to\mathbb{C}$, $1_G(x) = 1$ for all $x\in G$, is isolated in $\hat{G}$ with the Fell topology. Bekka took ...

0
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0
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54
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### Are banach space representations of commutative $C^*$ algebras decomposable?

It is well known that, if $\pi:A\to \mathbb B(\mathcal H)$ is a $^*$-representation of a type I $C^*$-algebra, then $\pi$ is unitarily equivalent to a direct integral of irreducible representations.
...

0
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0
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100
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### Semisimplicity of certain quotients of commutative Banach algebras

Let $A$ be a (Jacobson) semisimple regular commutative Banach algebra with a bounded approximate identity.
For a given $f\in A^{\ast}$, let $k_f= \lbrace a\in A: Aa\subseteq\ker{f}\rbrace$. It is not ...

1
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0
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46
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### Banach algebras satisfying $pq=qp=q \Rightarrow \|q\|\leq\|p\|$ for all idempotents $p$ and $q$

This question could be way below the level of MO, so apologies in advance. I posted the same question in MS about 10 days ago without a definitive answer so far.
Let $A$ be a Banach algebra with the ...

2
votes

1
answer

251
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### Qualitative difference between "continuous" and "discontinuous" states on $M(G)$

Let $G$ be a locally compact Abelian group (we can think that $G={\mathbb R}$). Let $C_0(G)$ be the space of continuous functions $u:G\to{\mathbb C}$ vanishing at infinity with the usual $\sup$-norm, ...

5
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248
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### Associativity of the Campbell-Baker-Hausdorff operation on a Banach-Lie algebra

Let $(\mathfrak{g}, [\cdot,\cdot]_\mathfrak{g}, \Vert \cdot \Vert_\mathfrak{g})$ be an infinite-dimensional Banach-Lie algebra, and let us define for any $a,b \in \mathfrak{g}$ the series
$$~ Z^\...

0
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0
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58
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### Boolean algebras generated by order intervals of projectional skeletons

A projectional skeleton (PS) on a Banach space $X$ is a family of projections $\mathcal{P}=\{P_i : i\in J\}$, indexed by a directed and $\sigma$-complete set $(J,\leq)$, satisfying
$P_i(X)$ is ...

1
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0
answers

89
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### Module homomorphisms modulo compact operators

Let $A$ be a Banach algebra. Let $L_a,R_a:A\to A$ denote the left/right multiplication operators $$L_ax = ax, \hspace{5mm} R_ax = xa$$ for all $a,x\in A$. Assume that no nonzero $L_a$ and $R_a$ is a ...

2
votes

0
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### An square root of the multiplicative operator on $\ell^1(\mathbb{Z}_n)$

Let us consider the finite group algebra $\ell^1(\mathbb{Z}_n)$. Let $x=(x_0,\cdots,x_{n-1})$ in $\ell^1(\mathbb{Z}_n)$ and define
$$M_x: \ell^1(\mathbb{Z}_n)\to \ell^1(\mathbb{Z}_n) : M_x(a)=a*x$$
...

0
votes

1
answer

79
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### Do the weakly null sequences in a Banach module factor?

Let $A$ be a Banach algebra with a bounded approximate identity, and let $E$ be a Banach left $A$-module. Suppose neither $A$ nor $E$ has the Schur property.
Question: Given a weakly null sequence $(...

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0
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35
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### Regarding significance of spectral variation under algebraic operations

I have been reading the paper Determining elements in $C^∗$-algebras through spectral properties.
The paper discusses about what would be the relation be between two elements $a$ and $b$ of a Banach ...

2
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138
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### Solvability and nilpotency for Banach algebras

Do we have topological counterparts of solvability and nilpotency, which are central concepts for (finite-dimensional) Lie algebras, for infinite dimensional Banach algebras with the commutator ...

1
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0
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189
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### Contractive projections on operator algebras

This is a follow up on an earlier question.
In [Lau&Loy, 2008] a Banach algebra $\mathcal{U}$ was called to have the Tomiyama property if any contractive projection $P:\mathcal{U}\to \mathcal{U}$, ...

2
votes

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### A closed ideal in $L^1(T)$

Let $\mathbb{T}$ be the unit circle and consider the convolution group algebra $L^1(\mathbb{T})$. Let $I_n$ be the closed ideal generated by the polynomial $p_n(z)=z^n-1$ in $L^1(\mathbb{T})$.
Let $I=...

2
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0
answers

79
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### Banach algebras for which left invertible implies invertible

Are there noncommutative Banach algebras in which left invertibility implies invertibility? If so, what are they called?

1
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1
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108
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### Regarding variation of spectra

I have been reading the article The variation of spectra by J.D Newburgh. in this article and all related reference/ articles, the term 'variation of spectra' keeps coming in, but I nowhere find a ...

1
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0
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108
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### The functional calculus on continuous functions

Let us consider $C[0,1]$ the space of continuous functions on the closed unit interval. For a given $x$ in $C[0,1]$, let us consider $A(sp(x))$, all analytic functions on a neighborhood of $sp(x)$.
...

5
votes

1
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300
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### Permanent invertible elements

Let $A$ be a unital complex algebra with the unit $\bf1$. Let $\mathcal{N}$ be the family of all norms on $A$ making it a unital normed algebra with the same unit $\bf1$. Let us put
$B_{\|\cdot\|}...

2
votes

1
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### Two completely different norms on a unital algebra!

Does there exist any unital normed algebra $(A,\|\cdot\|)$ enjoying another norm $\|\cdot\|_1$ such that
$(A,\|\cdot\|_1)$ forms a unital normed algebra with the same unit.
Any element contained ...

4
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0
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### Are fibers in the corona of $H^\infty$ separable?

Let $\mathcal{M}(H^\infty(\mathbb{D}))$ denote the spectrum of the Banach algebra $H^\infty$ and $\mathcal{M}_z(H^\infty(\mathbb{D}))$ the fiber over $z\in \mathbb{D}$, i.e. $\{\varphi\in \mathcal{M}:...

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### $ f,g\in \mathrm{VMO} $ but $ f\cdot g\notin \mathrm{VMO} $

We say a function $ f\in L^1_{\mathrm{loc}}(\mathbb{R}) $ is in $\mathrm{BMO}(\mathbb{R})$ if
$$\|f\|_{\mathrm{BMO}}=\sup_{I}\frac{1}{|I|}\int\limits_I |f(y)-f_I|\, dy<\infty$$ for all intervals $I\...

2
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1
answer

142
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### Is a certain property of a continuous map preserved under "surjectification"?

Let $X$ and $Y$ be compact Hausdorff spaces and let $\varphi:X\to Y$ be continuous with a property that if $A$ is a nowhere dense zero-set in $Y$, then $\varphi^{-1}(A)$ is nowhere dense in $X$. Let $...

3
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0
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127
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### Unital commutative dual Banach *-algebras whose $w^*$-closed ideals are principal

Let $A$ be a commutative Banach *-algebra. For a given ideal $I$ of $A$, we say that, it is principal if there is a projection $p$ (i.e. $p^2=p=p^*$) in $A$ with $I=Ap$.
Q. Any characterization ...

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### Are nearby subalgebras of matrix algebras conjugate?

Let $k=\mathbb{R}$ or $\mathbb{C}$ and let $A$ be a finite-dimensional $k$-algebra. If $A$ is simple, then the Skolem-Noether theorem says that any two algebra homomorphisms $f, g: A \to M_n(k)$ are ...

7
votes

1
answer

249
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### Does every infinite-dimensional Banach algebra contain an infinite-dimensional subalgebra with second-countable primitive ideal space?

Let $A$ be an infinite dimensional Banach algebra. Even if separable the primitive ideal space of $A$ need not be second-countable when endowed with the hull-kernel topology. Can we at least find an ...

3
votes

0
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131
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### Non-emptiness of spectrum $\sigma(a)$ in non-Archimedean Banach algebras

I'm trying to determine whether or not the standard proof that the spectrum of a point in a unital Banach algebra is non-empty can be adapted to prove the same thing over certain non-Archimedean ...

2
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0
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146
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### A generalisation of closed and bounded subsets of non-Archimedean fields to topological spaces

The definition of compactness in topological spaces generalises the notion of a subset of $\mathbb{R}^n$ being closed and bounded, as expressed by the Heine-Borel Theorem.
In finite-dimensional vector ...

3
votes

0
answers

269
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### Left ideals of $\ell^{\infty}(A)$ containing all weakly null sequences in a Banach algebra $A$

Let $A$ be a Banach algebra. $\ell^{\infty}(A)$, the space of all bounded sequences in $A$, is a Banach algebra with pointwise operations.
Let $w_0(A)$ be the subspace of all weakly null sequences in $...

4
votes

1
answer

163
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### If $A\hat\otimes B$ has identity then so are $A$ and $B$

Let $A$ and $B$ be commutative Banach algebra. I have proven that if $A$ and $B$ have identity $e_A$ and $e_B$ respectivly , then $e_A\hat\otimes e_B$ is identity for $A\hat\otimes B$ (the ...

2
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132
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### $B(X)$ and $B(X)^{**}$ for superreflexive Banach spaces $X$

Let $A$ be a Banach algebra. It is well-known that $A^{**}$ with either Arens product has separately weak$^{*}$ continuous multiplication if and only if $A$ is Arens regular [Theorem 1, ...

0
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2
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223
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### Projectional skeletons in dual Banach algebras

A Banach algebra $A$ is a dual Banach algebra if it is a dual Banach space with a (not necessarily unique) predual $A_{\ast}$, and the multiplication on $A$ is separately weak*-continuous. Dual Banach ...