On free lunches in random walk markets with short-sale constraints and small transaction costs, and weak convergence to Gaussian continuous-time processes

TL;DR

This paper investigates conditions under which discrete-time random walk markets with constraints approximate continuous Gaussian processes, highlighting when arbitrages or free lunches can occur and their relation to small transaction costs.

Contribution

It provides new criteria linking discrete market models with their continuous limits, especially under short-sale constraints and small transaction costs, including fractional Brownian motion cases.

Findings

Identifies conditions for arbitrage and free lunches in discrete markets.

Shows convergence of discrete models to Gaussian continuous processes.

Demonstrates arbitrage-free conditions under small transaction costs for fractional Brownian motion.

Abstract

This paper considers a sequence of discrete-time random walk markets with a safe and a single risky investment opportunity, and gives conditions for the existence of arbitrages or free lunches with vanishing risk, of the form of waiting to buy and selling the next period, with no shorting, and furthermore for weak convergence of the random walk to a Gaussian continuous-time stochastic process. The conditions are given in terms of the kernel representation with respect to ordinary Brownian motion and the discretisation chosen. Arbitrage and free lunch with vanishing risk examples are established where the continuous-time analogue is arbitrage-free under small transaction costs - including for the semimartingale modifications of fractional Brownian motion suggested in the seminal Rogers (1997) article proving arbitrage in fBm models.