Problem bank

Problem 326 of 333MediumStochastic CalculusP326

Volatility of volatility and the square root

  1. Under the pricing measure, a stock's instantaneous variance vtv_t follows a driftless geometric Brownian motion,

    dvt=ξ vt dWt,v0=0.04,dv_t = \xi\,v_t\,dW_t, \qquad v_0 = 0.04,

    so volatility starts at 20%. The volatility of variance is ξ=0.8\xi = 0.8, with time in years.

    (a) Use Itô's lemma to find the SDE for the volatility σt=vt\sigma_t = \sqrt{v_t}. What are its drift and its volatility?

    (b) Find E[σ1]E[\sigma_1] and compare it with E[v1]\sqrt{E[v_1]}.

    (c) Find the median of σ1\sigma_1. A trader argues that variance is a fair game here, so volatility must be one too. What is wrong with the argument?