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Distribution and SDE of a ratio of two Brownian motions

Question

Let WW and ZZ be independent standard Brownian motions and Xt=Wt/ZtX_t=W_t/Z_t. What is the distribution of XtX_t for fixed t>0t>0, and what SDE does XX satisfy?

Short answer

Standard Cauchy at every tt. WtW_t and ZtZ_t are independent N(0,t)N(0,t), and tt cancels in the ratio of two i.i.d. centred normals. Using d(1/Z)=−Z−2dZ+Z−3dtd(1/Z)=-Z^{-2}dZ+Z^{-3}dt and dW dZ=0dW\,dZ=0, Ito gives dXt=XtZt2dt+1ZtdWt−XtZtdZtdX_t=\tfrac{X_t}{Z_t^2}dt+\tfrac{1}{Z_t}dW_t-\tfrac{X_t}{Z_t}dZ_t (valid while Z≠0Z\neq0). The noise can be written as 1+Xt2Zt dBt\tfrac{\sqrt{1+X_t^2}}{Z_t}\,dB_t for a single Brownian motion BB.

Adapted from Joshi, Denson & Downes, Quant Job Interview Questions and Answers, Q3.48, p.85; question and answer rewritten by SelfMadeQuant.com.

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