Wednesday, December 6, 2017

risk neutral measure - Version of Girsanov theorem with changing volatility


Is there a version of Girsanov theorem when the volatility is changing?


For example Girsanov theorem states that Radon Nikodym (RN) derivative for a stochastic equation is used to transform the expectation where the sampling is done in one mesaure to an expectation where sampling is done in another measure.


Let's see an example


dXt(w)=f(Xt(w))dt+σ(Xt(w))dWPt(w) in P measure.


In P* measure, drift is f(Xt(w)). We multiply the internals of expectation in P measure with RN derivative to get expectation of X in P* measure


EP[X]=EP[XdPdP]


where


dPdP=e0.5(f(Xs(w))f(Xs(w))σ(Xs(w)))2ds+f(Xs(w))f(Xs(w))σ(Xs(w))dWPs(w)


What I am looking for is in P* measure, not only drift but also the volatility changes



dXt(w)=f(Xt(w))dt+σ(Xt(w))dWPt(w)


Then what is dPdP?




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