MATH0085 - Asset Pricing in Continuous Time
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MATH0085 - Asset Pricing in Continuous Time
Problem 1. Let (Ω ; F; {Ft }t0 ; P) be some probability space and {Wt }t0 and {Wˆt }t0 be two (P; {Ft }t0)-Brownian motions, where EP [(Wt − Ws )(Wˆt − Wˆs )] = (t − s) for some ∈ (0; 1).
(a) [8 points] [SS] Let {Yt }t0 and {Zt }t0 be Ito processes governed by
Yt = Y0 + as ds + bs dWs ; and Zt = Z0 + cs ds + ds dWˆs :
If St = sin(t) + Yt2 + exp(Zt ) for all t ≥ 0, governed by the stochastic differential equation dSt = Kt dt + Lt dWt + Nt dWˆt , what are Kt , Lt and Nt in terms of Yt , Zt , at , bt , ct and dt for all t ≥ 0?
(b) [8 points] [S]
From above, if Xt = Yt Zt for all t ≥ 0 such that dXt = t dt + t dMt , where {Mt }t0 is a (P; {Ft }t0)-Brownian motion, what are t and t in terms of Yt , Zt , at , bt , ct and dt for all t ≥ 0?
(c) [9 points] [SS]
Define {Pt }0t< by Pt = sin(Wt )= cos(Wt ), where = inf{t ≥ 0 : |Wt | = }. Show that the following holds:
Pt = Ps (1 + P ) ds +s(2) (1 + Ps(2)) dWs :
Problem 2.
(a) [8 points] [SS]
Let (Ω ; F; {Ft }t0 ; P) be some probability space and {Wt }t0 be a (P; {Ft }t0)- Brownian motion. For a finite > 0, let {Ft }t0 , {Gt }t0 and {Ht }t0 be
Ft = cosh(Wt ) exp ( )
Gt = + Wt3 − 3tWt + Wt2 − t
Ht = Wt
for all t ≥ 0, respectively. For each above, prove whether it is a (P; {Ft }t0)- martingale or not.
(b) [8 points] [S]
Let {Bt }t0 given by Bt = ert model the money-market account for some finite r ≥ 0 and let {Zt }t0 be governed by dZt = −t Zt dWt where {t }t0 is the market price of risk. What is the stochastic differential equation of {t }t0 given by t = Zt =Bt ? Also, prove that {t }t0 is a (P; {Ft }t0)- supermartingale.
(c) [9 points] [U]
Let 0 be some finite constant an {Et }0t< be governed by the following:
Et = + Es(3) ds − Es(2) dWs ; E0 = :
Provide an explicit solution to the stochastic integral above – that is, what is the function f if the solution is Et = f (t; Wt ). Based on this solution, what should be the definition of in terms of Wt and ?
Problem 3. Let (Ω ; F; {Ft }t0 ; P) be some probability space and {Wt }t0 be a (P; {Ft }t0)-Brownian motion. Let {Bt }t0 governed by dBt = rBt dt model the money-market account, where B0 = 1 and r > 0 is finite. Let {Xt }t0 model asset price dynamics where dXt = Xt dt + Xt dWt for some finite constants ∈ R and > 0 with X0 = x.
(a) [8 points] [S]
Prove that {Zt }t0 defined by
Zt = exp ( − 2 t − Wt )
is a (P; {Ft }t0)-martingale if = ( − r)= .
(b) [8 points] [SS]
Define the risk-neutral probability measure Q as follows:
FT = exp ( − 2 T − WT ) :
For some > 0 and K ≥ 0, derive the following option price: V0 = e rTEQ [(X − K)+]. (Hint: These are called power options)
(c) [9 points] [SS]
Now define the probability measure U as follows:
dU XT
dQ FT xBT
Show that for any 0 ≤ s ≤ t ≤ T , the following holds:
EU [ Fs] =
Problem 4. Let (Ω ; F; {Ft }t0 ; Q) be a probability space where Q is the risk- neutral probability measure and {Wt }t0 is a (Q; {Ft }t0)-Brownian motion. For interest rate r > 0 and volatility > 0 (both finite), let {Xt }t0 be governed by dXt = rXt dt + Xt dWt .
(a) [9 points] [U] If g(x) is a non-negative convex function for x ≥ 0 with g(0) = 0, prove that {St }t0 given by St = e rtg(Xt ) is a (Q; {Ft }t0)- submartingale. (Hint: Since g is convex, g ((1 − )x1 + x2 ) ≤ (1−)g(x1 )+ g(x2 ) for 0 ≤ ≤ 1 and 0 ≤ x1 ≤ x2 . Set x1 = 0 and x2 = x)
(b) [8 points] [S]
Define the stochastic integral process {Mt }t0 as follows:
Mt = Wt3 − Ws ds:
Compute the expected value EQ [Mt ] and the variance VarQ [Mt ].
(c) [8 points] [SS]
Let {Wˆt }t0 be another (Q; {Ft }t0)-Brownian motion which is independent of {Wt }t0 . Define Yt = Wt= 2 for some > 0 and t = tWˆ1=t for t ≥ 0, with Y0 = 0 = 0. If Rt = Yt =t whenever t 0, what is the distribution Q(R1 ∈ dr)?
2022-04-27