Let W(t) be a standard Brownian motion. Let a > 0. Define Ta as the first time

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Let W(t) be a standard Brownian motion. Let a > 0. Define Ta as the first time that W(t) = a. That isTa = min{t: W(t) = a}.

a. Show that for any t ≥ 0, we haveP(W(t) a) = P(W(t)  aTa t)P(Ta t).

b. Using Part (a), show thatP(Ta t)=21-0 a (+7)] t

c. Using Part (b), show that the PDF of Ta is given byfr. (t) = a t/2t p{-} exp 2t

By symmetry of Brownian motion, we conclude that for any a ≠ 0, we havefra (t) = lal t2t exp 2t

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