# The next result will lead to a number of examples

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Unformatted text preview: ing Y by 1A sets the product = 0 on Ac and leaves the values on A unchanged. Finally, we deﬁne the conditional expectation of Y given A to be E (Y |A) = E (Y ; A)/P (A) This is the expected value for the conditional probability deﬁned by P (·|A) = P (· \ A)/P (A) Example 5.1. A simple but important special case arises when the random variable Y and the set A are independent, i.e., for any set B we have P (Y 2 B, A) = P (Y 2 B )P (A) Noticing that this implies that P (Y 2 B, Ac ) = P (Y 2 B )P (Ac ) and comparing with the deﬁnition of independence of random variables in (A.13), we see that this holds if and only Y and 1A are independent, so Theorem A.1 implies E (Y ; A) = E (Y 1A ) = EY · E 1A 159 160 CHAPTER 5. MARTINGALES and we have E (Y |A) = EY (5.1) It is easy to see from the deﬁnition that the integral over A is linear: E (Y + Z ; A) = E (Y ; A) + E (Z ; A) (5.2) so dividing by P (A), conditional expectation also has this property E (Y + Z |A) = E (Y |A) + E (Z |A) (5.3) Here and in later formulas and theorems, we...
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## This document was uploaded on 03/06/2014 for the course MATH 4740 at Cornell University (Engineering School).

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