Volume 30, Issue 4 p. 1527-1564
ORIGINAL ARTICLE

A martingale representation theorem and valuation of defaultable securities

Tahir Choulli

Corresponding Author

Tahir Choulli

Department of Mathematical and Statistical Sciences, University of Alberta, Edmonton, Canada

Correspondence

Tahir Choulli, Department of Mathematical and Statistical Sciences, University of Alberta, 632 Central Academic Building, Edmonton, Canada.

Email: tchoulli@ualberta.ca

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Catherine Daveloose

Catherine Daveloose

Department of Applied Mathematics, Computer Science and Statistics, Ghent University, Ghent, Belgium

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Michèle Vanmaele

Michèle Vanmaele

Department of Applied Mathematics, Computer Science and Statistics, Ghent University, Ghent, Belgium

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First published: 08 April 2020
Citations: 5

Funding information: NSERC (through grant NSERC RGPIN04987).

Abstract

We consider a financial framework with two levels of information: the public information generated by the financial assets, and a larger flow of information that contains additional knowledge about a random time. This random time can represent many economic and financial settings, such as the default time of a firm for credit risk, and the death time of an insured for life insurance. As the random time cannot be seen before its occurrence, the progressive enlargement of filtration seems tailor-fit to model the larger flow of information that incorporates both the public flow and the information about the random time. In this context, our interest focuses on the following challenges: (a) How to single out the various risks coming from the financial assets, the random time, and their correlations? (b) How these risks interplay and lead to the formation of any risk in the larger flow of information? It is clear that understanding how risks build-up and interact, when one enlarges the flow of information, is vital for an efficient risk management and derivatives' evaluation in those informational markets. Our answers to these challenges are full and complete no matter what the model for the random time is and no matter how the random time is related to the public flow. In fact, we introduce “pure default” risks, and quantify and classify these risks afterward. Then we elaborate our martingale representation results, which state that any martingale in the large filtration stopped at the random time can be decomposed into orthogonal local martingales (i.e., local martingales whose product remains a local martingale). This constitutes our first principal contribution, while our second contribution consists in evaluating various defaultable securities according to the recovery policy, within our financial setting that encompasses any default model, using a martingale “basis.” Our pricing formulas explain the impact of various recovery policies on securities and determine the types of pure default risk they entail.

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