Problem bank

Problem 325 of 333MediumStochastic CalculusP325

How often does a trend follower lose?

  1. An asset's price, measured in price units from where it starts, is Xt=μt+WtX_t = \mu t + W_t for t∈[0,1]t \in [0, 1], where WW is a standard Brownian motion. A trend follower holds XtX_t units at time tt: long when the price is above its starting level, short when it is below, rebalancing continuously. Each position is set before the next price move, so the P&L is the Itô integral

    Π=∫01Xt dXt.\Pi = \int_0^1 X_t\,dX_t.

    (a) Write Π\Pi in terms of X1X_1 alone, and find E[Π]E[\Pi] as a function of μ\mu.

    (b) Take μ=0\mu = 0. What is the probability that the strategy loses money? What is the largest possible loss? What are the average loss given a loss and the average gain given a gain?

    (c) Now the asset trends, with μ=1\mu = 1 or μ=−1\mu = -1. What are E[Π]E[\Pi] and the probability of a loss?