On the Bohnenblust-Hille inequality and a variant of Littlewood's 4/3 inequality

Abstract

The search for sharp constants for inequalities of the type Littlewood's 4/3 and Bohnenblust-Hille has lately shown unexpected applications in many fields such as Analytic Number Theory, Quantum Information Theory, or in results on n-dimensional Bohr radii. Recent estimates obtained for the multilinear Bohnenblust-Hille inequality (for real scalars) have been used, as a crucial tool, by A. Montanaro in order to solve problems in Quantum XOR games. Here, among other results, we obtain new upper bounds for the Bohnenblust-Hille constants (for complex scalars). For bilinear forms, we provide optimal constants of variants of Littlewood's 4/3 inequality (for real scalars) when the exponent 4/3 is replaced by any r >= 4/3. We also prove that the optimal constants in real case are always strictly greater than those from the complex case.