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Register a new userSlicely Countably Determined Banach spaces
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We introduce the class of slicely countably determined Banach spaces which contains in particular all spaces with the RNP and all spaces without copies of $ell_1$. We present many examples and several properties of this class. We give some applications to Banach spaces with the Daugavet and the alternative Daugavet properties, lush spaces and Banach spaces with numerical index 1. In particular, we show that the dual of a real infinite-dimensional Banach with the alternative Daugavet property contains $ell_1$ and that operators which do not fix copies of $ell_1$ on a space with the alternative Daugavet property satisfy the alternative Daugavet equation.
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A Banach algebra $A$ is said to be a zero Jordan product determined Banach algebra if every continuous bilinear map $varphicolon Atimes Ato X$, where $X$ is an arbitrary Banach space, which satisfies $varphi(a,b)=0$ whenever $a$, $bin A$ are such that $ab+ba=0$, is of the form $varphi(a,b)=sigma(ab+ba)$ for some continuous linear map $sigma$. We show that all $C^*$-algebras and all group algebras $L^1(G)$ of amenable locally compact groups have this property, and also discuss some applications.
A Banach algebra $A$ is said to be zero Lie product determined if every continuous bilinear functional $varphi colon Atimes Ato mathbb{C}$ with the property that $varphi(a,b)=0$ whenever $a$ and $b$ commute is of the form $varphi(a,b)=tau(ab-ba)$ for some $tauin A^*$. In the first part of the paper we give some general remarks on this class of algebras. In the second part we consider amenable Banach algebras and show that all group algebras $L^1(G)$ with $G$ an amenable locally compact group are zero Lie product determined.
$C^*$-algebras, group algebras, and the algebra $mathcal{A}(X)$ of approximable operators on a Banach space $X$ having the bounded approximation property are known to be zero product determined. We are interested in giving a quantitative estimate of this property by finding, for each Banach algebra $A$ of the above classes, a constant $alpha$ with the property that for every continuous bilinear functional $varphicolon A times Atomathbb{C}$ there exists a continuous linear functional $xi$ on $A$ such that [ sup_{Vert aVert=Vert bVert=1}vertvarphi(a,b)-xi(ab)vertle alphasup_{mathclap{substack{Vert aVert=Vert bVert=1, ab=0}}}vertvarphi(a,b)vert. ]
A Banach algebra $A$ is said to be zero Lie product determined if every continuous bilinear functional $varphi colon Atimes Ato mathbb{C}$ satisfying $varphi(a,b)=0$ whenever $ab=ba$ is of the form $varphi(a,b)=omega(ab-ba)$ for some $omegain A^*$. We prove that $A$ has this property provided that any of the following three conditions holds: (i) $A$ is a weakly amenable Banach algebra with property $mathbb{B}$ and having a bounded approximate identity, (ii) every continuous cyclic Jordan derivation from $A$ into $A^*$ is an inner derivation, (iii) $A$ is the algebra of all $ntimes n$ matrices, where $nge 2$, over a cyclically amenable Banach algebra with a bounded approximate identity.
We prove that every isometry between two combinatorial spaces is determined by a permutation of the canonical unit basis combined with a change of signs. As a consequence, we show that in the case of Schreier spaces, all the isometries are given by a change of signs of the elements of the basis. Our results hold for both the real and the complex cases.
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