Sharp bounds for products of dependent random variables

TL;DR

This paper derives sharp bounds for the expectation of the product of dependent random variables with known marginals but unknown dependence, using a novel decomposition into magnitude and sign components.

Contribution

It introduces a decomposition approach and parity polytope conditions to determine universal bounds and construct extremal couplings for dependent variables.

Findings

Universal bounds depend on the parity polytope conditions.

Extremal couplings are constructed via measurable selections on the parity polytope.

Explicit extremal copulas are provided for three dimensions.

Abstract

We study the sharp bounds of $\mathbb{E}[X_1\cdots X_d]$ when the univariate marginal distributions are known, but the dependence structure between them is unspecified. Maximizing products over non-negative variables is straightforward via the comonotonic coupling, but the problem is more subtle when the marginals can take both positive and negative values. Specifically, two negative realizations can be matched to yield a positive product, whereas a single negative realization necessarily yields a negative product. We propose a decomposition of the problem into a magnitude part and a sign part, and show that universal upper and lower bounds for the product expectation follow from the comonotonic coupling of the absolute values and properly chosen sign vectors. Under a mild regularity assumption, we give necessary and sufficient conditions for these universal bounds to be attainable. For the upper bound, the marginal sign-bias vector must belong to the even-parity polytope, while for the lower, the corresponding condition involves the odd-parity polytope. We construct the extremal couplings via measurable selections on the parity polytope whenever these conditions hold. We study the case of identical marginals in more detail and provide examples of non-symmetric extremal coupling that achieve the universal bounds. We explicitly construct the extremal copulas in three dimensions, and use a recursive parity decomposition to obtain higher-dimensional extremal copulas from the trivariate ones.