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From Principle-Based Risk Management to Solvency ... - Scor

From Principle-Based Risk Management to Solvency ... - Scor

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We now return <strong>to</strong> the notion of perfect replication. We can distinguish<br />

two types of replicating portfolios: static portfolios, where the replicating<br />

portfolio is set up at the beginning and kept constant over time (although,<br />

of course, its value probably changes over time); and dynamic portfolios,<br />

where the initial portfolio is adjusted over time in a self-financing way (i.e.<br />

with no in- or out-flow of money, with the exception of the liability cash<br />

flows themselves) as information is revealed.<br />

Uniqueness of the value derived by perfect replication follows from a noarbitrage<br />

argument: If two portfolios of liquidly traded financial instruments<br />

perfectly replicate a given instrument, their values have <strong>to</strong> be equal, because<br />

otherwise an arbitrage opportunity would exist. To be more precise, because<br />

the market is liquid, the two portfolios can be bought and sold for their value<br />

(which is the market value), so the theoretical arbitrage opportunity can be<br />

realized.<br />

We mention two well-known examples of valuation by perfect replication.<br />

As a first example, consider the value at time t of a deterministic future<br />

payment at time t + d. This instrument is perfectly replicated by a defaultfree<br />

zero-coupon bond with maturity d. Hence, the value of the future<br />

payment is equal <strong>to</strong> the price of the bond at time t. The replicating portfolio<br />

is clearly static.<br />

As a second example, consider, at time t, a s<strong>to</strong>ck option with strike date<br />

t + d. The cash flow at t + d of the option is s<strong>to</strong>chastic, and depends on<br />

the value of the s<strong>to</strong>ck at t + d. Nonetheless, option pricing theory shows<br />

that, under certain model assumptions, the option cash flow at t + d can<br />

be perfectly replicated by holding at time t a portfolio composed of certain<br />

quantities (long or short) of cash and the s<strong>to</strong>ck, and by dynamically adjusting<br />

this portfolio over time in a self-financing way. So this is a dynamic<br />

replicating portfolio. The value of the option at time t is again equal <strong>to</strong> the<br />

price of the replicating portfolio at t.<br />

In this example, the s<strong>to</strong>chasticity of the cash flow becomes irrelevant,<br />

since perfect replication means that replication works for any state of the<br />

world. Assuming the option is part of a larger portfolio of financial instruments,<br />

its value is independent of the other instruments in the portfolio,<br />

on how their cash flows depend on each other, and on the volume of the<br />

portfolio.<br />

Observe that particularly the second example is not realistic in that it<br />

does not take in<strong>to</strong> account transaction costs, which are inevitably part of the<br />

replication process. Crucially, when taking in<strong>to</strong> account transaction costs,<br />

the costs of the replication are likely no longer independent of the overall<br />

portfolio. It might be, for instance, that certain transactions needed <strong>to</strong><br />

dynamically adjust the replicating portfolio for different instruments cancel<br />

each other out, so that overall transaction costs are reduced. Therefore, the<br />

value of one instrument depends on the overall portfolio of instruments and<br />

is no longer unique - even though the replication is perfect and s<strong>to</strong>chasticity<br />

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