STAT0013 STOCHASTIC METHODS IN FINANCE 2022–23
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STOCHASTIC METHODS IN FINANCE 2022–23
STAT0013
Exercises 6 - Stochastic Calculus
1. Find the stochastic differential equation (SDE) satisfied by the square of a stock price that follows geometric Brownian motion. What is this process?
2. (a) Suppose that g(t) is a deterministic differentiable function for t > 0, with g(0) = 0. Show that a solution to the ordinary differential equation
dx(t) = ax(t)dt + bx(t)dg(t)
with boundary condition x(0) = x0 0 is
x(t) = x0 eat+bg(t) .
Hint: Write dg(t) as g\ (t)dt and then use the variable separation technique and take integrals on both sides of the equation.
(b) Show that the process Xt = xeat+bWt , where Wt is standard Brow- nian motion, satisfies the SDE
dXt = (a + )Xt dt + bXt dWt
with initial condition X0 = x.
3. Show that the Itˆo process Xt = eWte−t/2 (with Wt a standard Brownian
motion) satisfies the stochastic differential equation
dXt = Xt dWt .
4. A zero-coupon government bond pays £100 at time T, and has price denoted by Bt . In the course so far we have assumed that the risk- free rate is constant and deterministic. In more advanced models, the risk-free rate can be modelled itself as a stochastic process. It has been suggested that the short-term interest rate, rt , will not be constant over time but will in fact follow the stochastic process
drt = a(b − rt )dt + crt dzt
where a, b, c are positive constants and zt is a standard Brownian motion. Under this assumption, derive the SDE for the government bond price Bt for t < T.
2023-05-07
Stochastic Calculus