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E (Mn+1 Mn Av ) = Mn E (Xn+1 1Av ) = 0 The reason for a multiplicative model is that changes in stock prices are thought
to be proportional to its value. Also, in contrast to an additive model, we are
guaranteed that prices will stay positive.
The last example generalizes easily to give:
Example 5.6. Exponential martingale. Let Y1 , Y2 , . . . be independent and
identically distributed with (✓) = E exp(✓Y1 ) < 1. Let Sn = S0 + Y1 + · · · + Yn .
Then Mn = exp(✓Sn )/ (✓)n is a martingale with respect to Yn . In particular,
if (✓) = 1 then (✓Sn ) is a martingale.
Proof. If we let Xi = exp(✓Yi )/ (✓) then Mn = M0 X1 · · · Xn with EXi = 1
and this reduces to the previous example.
Having introduced a number of examples, we will now derive some basic
properties.
Lemma 5.6. If Mn is a martingale and is a convex function then (Mn )
is a submartingale. If Mn is a submartingale and is a nondecreasing convex
function then (Mn ) is a submartingale.
Proof. Using Lemma 5.2 and the deﬁnition of a martingale, we have
E ( (Mn+1 )Av ) (E (Mn+1 Av )) = (Mn ) In the proof of the second statem...
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This document was uploaded on 03/06/2014 for the course MATH 4740 at Cornell University (Engineering School).
 Spring '10
 DURRETT
 The Land

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