STATS 723 - Stochastic methods in finance - 2022 Assignment 1
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STATS 723 - Stochastic methods in finance - 2022
Assignment 1
1. Give an example of a sample space Ω. Find two different probability measures P and Q on Ω. Define a random variable X on Ω, and give both distributions of X: the distribution when P-probabilities are used (a.k.a. the distribution “under P”) and the distribution under Q.
2. Let X0 , X1 , X2 , . . . be a simple random walk. That is, X0 = 0 and Xk = i(k)=1 Yi where Y1 , Y2 , . . . are independent and take values .1 with probability each. Only finitely many things can happen in the first four steps, so each of the following σ-fields partitions the sample space into finitely many parts.
i σ(X1 )
ii σ(X4 )
iii σ(X1 , X4 )
iv σ(X0 , . . . , X4 )
v σ (max(X0 , . . . , X4 ))
For each σ-field, find
(a) What the parts are. Example. σ(X1 ) partitions the sample space into two parts: |X1 = 1| and |X1 = ← 1|. (b) The conditional expectation of X4 given the information represented by the σ-field.
3. Let Y and Z be independent random variables, with Z having the N(0, 1) distribution and Y having the distribution with pdf f(y) = e −y for y > 0 (a.k.a. the exponential distribution). Use a double integral to calculate P (Y + 2Z > 0).
4. Let X0 , X1 , X2 , . . . be a standard normal random walk. That is, X0 = 0 and Xk = i(k)=1 Zi where Z1 , Z2 , . . . are independent and distributed N(0, 1). Let (rn ) be the usual filtration: rn = σ(X0 , . . . , Xn ).
(a) Demonstrate that X1 and X2 are not independent by finding examples of non-independent events in r1 and σ(X2 ).
(b) For each of the following random variables, determine, firstly, whether the random variable is measurable with respect to rn , and secondly, the conditional expectation of the random variable given the information represented by rn .
i. Zn+1
ii. Xn
iii. Xn − 1
iv. Xn+1
v. X1 + X2 + 一 一 一 + X2n
vi. maxXi
vii. Xn(3)Zn(2)+1
(c) Show that the sequence (Yn ) given by Yn = (Xn(2) ← 3n)Xn is a martingale.
(d) Hence, or otherwise, find an expression for E ┌Xn(3) │rm ┐ when m 士 n.
2022-03-15