On bivariate lower semilinear copulas and the star product

TL;DR

This paper investigates the properties of bivariate lower semilinear (LSL) copulas, proving their closure under the star product, analyzing their limits and idempotent elements, and exploring their concordance measures and the structure of their measure region.

Contribution

The paper establishes that the family of LSL copulas is closed under the star product, characterizes the limits of their iterated star products, and analyzes their concordance measure region.

Findings

Star product of two LSL copulas is again LSL.

Sequences of iterated star products converge to an idempotent LSL copula.

The concordance measure region for LSL copulas is convex and compact.

Abstract

We revisit the family $\mathcal{C}^{LSL}$ of all bivariate lower semilinear (LSL) copulas first introduced by Durante et al. in 2008 and, using the characterization of LSL copulas in terms of diagonals with specific properties, derive several novel and partially unexpected results. In particular we prove that the star product (also known as Markov product) $S_{\delta_1}*S_{\delta_2}$ of two LSL copulas $S_{\delta_1},S_{\delta_2}$ is again a LSL copula, i.e., that the family $\mathcal{C}^{LSL}$ is closed with respect to the star product. Moreover, we show that translating the star product to the class of corresponding diagonals $\mathcal{D}^{LSL}$ allows to determine the limit of the sequence $S_\delta, S_\delta*S_\delta, S_\delta*S_\delta*S_\delta,\ldots$ for every diagonal $\delta \in \mathcal{D}^{LSL}$. In fact, for every LSL copula $S_\delta$ the sequence $(S_\delta^{*n})_{n \in \mathbb{N}}$ converges to some LSL copula $S_{\overline{\delta}}$, the limit $S_{\overline{\delta}}$ is idempotent, and the class of all idempotent LSL copulas allows for a simple characterization. Complementing these results we then focus on concordance of LSL copulas. After deriving simple formulas for Kendall's $\tau$ and Spearman's $\rho$ we study the exact region $\Omega^{LSL}$ determined by these two concordance measures of all elements in $\mathcal{C}^{LSL}$, derive a sharp lower bound and finally show that $\Omega^{LSL}$ is convex and compact.