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The Grothendieck inequality revisited

About this Title

Ron Blei, Department of Mathematics, University of Connecticut, Storrs, Connecticut 06269

Publication: Memoirs of the American Mathematical Society
Publication Year: 2014; Volume 232, Number 1093
ISBNs: 978-0-8218-9855-0 (print); 978-1-4704-1896-0 (online)
DOI: https://doi.org/10.1090/memo/1093
Published electronically: March 11, 2014
Keywords: The Grothendieck inequality, Parseval-like formulas, integral representations, fractional Cartesian products, projective continuity, projective boundedness
MSC: Primary 46C05, 46E30; Secondary 47A30, 42C10

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Table of Contents

Chapters

  • 1. Introduction
  • 2. Integral representations: the case of discrete domains
  • 3. Integral representations: the case of topological domains
  • 4. Tools
  • 5. Proof of Theorem 3.5
  • 6. Variations on a theme
  • 7. More about $\Phi$
  • 8. Integrability
  • 9. A Parseval-like formula for $\langle {\bf {x}}, {\bf {y}}\rangle$, ${\bf {x}} \in l^p$, ${\bf {y}} \in l^q$
  • 10. Grothendieck-like theorems in dimensions $>2$?
  • 11. Fractional Cartesian products and multilinear functionals on a Hilbert space
  • 12. Proof of Theorem 11.11
  • 13. Some loose ends

Abstract

The classical Grothendieck inequality is viewed as a statement about representations of functions of two variables over discrete domains by integrals of two-fold products of functions of one variable. An analogous statement is proved, concerning continuous functions of two variables over general topological domains. The main result is the construction of a continuous map $\Phi$ from $l^2(A)$ into $L^2(\Omega _A, \mathbb {P}_A)$, where $A$ is a set, $\Omega _A = \{-1,1\}^A$, and $\mathbb {P}_A$ is the uniform probability measure on $\Omega _A$, such that \begin{equation}\tag {1} \sum _{\alpha \in A} \textbf {{x}}(\alpha ) \bar {\textbf {{y}}(\alpha )} = \int _{\Omega _A} \Phi (\textbf {{x}})\Phi (\bar {\textbf {{y}}})d \mathbb {P}_A, \ \ \ \textbf {{x}} \in l^2(A), \ \textbf {{y}} \in l^2(A), \end{equation} and \begin{equation}\tag {2} \|\Phi (\textbf {{x}})\|_{L^{\infty }} \leq K \|\textbf {{x}}\|_2, \ \ \ \textbf {{x}} \in l^2(A), \end{equation} for an absolute constant $K > 1$. ($\Phi$ is non-linear, and does not commute with complex conjugation.) The Parseval-like formula in (1) is obtained by iterating the usual Parseval formula in a framework of harmonic analysis on dyadic groups. A modified construction implies a similar integral representation of the dual action between $l^p$ and $l^q$, $\frac {1}{p} + \frac {1}{q} = 1$.

Variants of the Grothendieck inequality in higher dimensions are derived. These variants involve representations of functions of $n$ variables in terms of functions of $k$ variables, $0 < k < n.$ Multilinear Parseval-like formulas are obtained, extending the bilinear formula in (1). The resulting formulas imply multilinear extensions of the Grothendieck inequality, and are used to characterize the feasibility of integral representations of multilinear functionals on a Hilbert space.

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