5.21 Given a probability space (S,F,P) and a random variable X. Let A be a sub-o-field of F, and consider  = E (X | A). (i) Show that E ((X – Â)Y) = 0 if Y is an A-measurable random vari- able. (ii) Let Y run over all A-measurable random variables. Show that the squared error E ((X – Y)²) is minimal when Y = X.

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5.21 Given a probability space (N, F, P) and a random variable X. Let A
be a sub-o-field of F, and consider Î = E(X | A).
(i) Show that E ((X – Â)Y) = 0 if Y is an A-measurable random vari-
able.
(ii) Let Y run over all A-measurable random variables. Show that the
squared error E ((X – Y)²) is minimal when Y = Â.
Transcribed Image Text:5.21 Given a probability space (N, F, P) and a random variable X. Let A be a sub-o-field of F, and consider Î = E(X | A). (i) Show that E ((X – Â)Y) = 0 if Y is an A-measurable random vari- able. (ii) Let Y run over all A-measurable random variables. Show that the squared error E ((X – Y)²) is minimal when Y = Â.
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