06.06.2013 Views

STOCHASTIC

STOCHASTIC

STOCHASTIC

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

CONSUMPTION AND PORTFOLIO RULES 393<br />

variable. 22 The following theorem shows that this result holds only if<br />

U(C, 0 C HARA(C).<br />

THEOREM V. Given the model specified in this section and the timedependent<br />

random variable Y(t) = U(C*, t), then Y is log-normally distributed<br />

if and only if U(C, t) C HARA(C).<br />

Proof. "If" part: it was previously shown that if U C HARA(C),<br />

then Y is log-normally distributed. "Only if" part: let C* = g(W, t) and<br />

w*W=f(W, t). By Ito's Lemma,<br />

dY = UcdC* + Utdt + \Ucc(dC*)\<br />

dC* = g„dW + gt dt + \gww(dW)\ (59)<br />

dW = [/(a -r) + rW-g]dt + afdz.<br />

Because (dW) 2 = a 2 / 2 dt, we have that<br />

dC* = [glrf(

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!