Abstract
For utility maximization problems under proportional transaction costs, it has been observed that the original market with transaction costs can sometimes be replaced by a frictionless shadow market that yields the same optimal strategy and utility. However, the question of whether or not this indeed holds in generality has remained elusive so far. In this paper, we present a counterexample which shows that shadow prices may fail to exist. On the other hand, we prove that short selling constraints are a sufficient condition to warrant their existence, even in very general multi-currency market models with possibly discontinuous bid–ask spreads.
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Notes
That is, a consistent price system in the terminology of Schachermayer [31].
Cf. Definition 3.9 below for the formal definition.
In the general multi-currency notation introduced below, this corresponds to [1/π 21,π 12], where π ij denotes the number of units of asset i for which the agent can buy one unit of asset j.
K(Π) contains precisely the solvent portfolios that can be liquidated to zero by trading according to the bid–ask matrix Π and possibly throwing away positive asset holdings.
In particular, we do not restrict ourselves to finite variation strategies here.
In fact, J can be seen as the restriction to \(\mathbb{R}^{d}_{+} \setminus \{0\}\) of some other concave function defined on K 0 that allows for negative initial endowment (but forces the agent to make an instantaneous trade at time 0 in that case).
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Acknowledgements
The authors are grateful to Bruno Bouchard, Christoph Czichowsky, Paolo Guasoni, Ioannis Karatzas, Marcel Nutz, Mark P. Owen, and Walter Schachermayer for fruitful discussions, and also acknowledge the constructive comments of two anonymous referees and the editor, Martin Schweizer. The second author thanks the “Chaire Les Particuliers Face aux Risques”, Fondation du Risque (Groupama-ENSAE-Dauphine), the GIP-ANR “Risk” project for their support. The fourth author was partially supported by the National Centre of Competence in Research “Financial Valuation and Risk Management” (NCCR FINRISK), Project D1 (Mathematical Methods in Financial Risk Management), of the Swiss National Science Foundation (SNF).
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Benedetti, G., Campi, L., Kallsen, J. et al. On the existence of shadow prices. Finance Stoch 17, 801–818 (2013). https://doi.org/10.1007/s00780-012-0201-4
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DOI: https://doi.org/10.1007/s00780-012-0201-4