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Related: Dumb question: is risk-neutral pricing taking conditional expectation?

In the one-step binomial model...


For $\frac{d \mathbb Q}{d \mathbb P}$,

I think it's $\frac{d \mathbb Q}{d \mathbb P} = \frac{q_u}{p_u}1_u + \frac{q_d}{p_d}1_d$, so it's some asset with payoffs $\frac{q_u}{p_u}$ and $\frac{q_d}{p_d}$, expected value of 1 and replicating portfolio $(x,y)$ of

$$x=\frac{1}{1+R}\frac{u(\frac{q_d}{p_d})-d(\frac{q_u}{p_u})}{u-d}$$ $$y=\frac{1}{S_0}\frac{\frac{q_u}{p_u}-\frac{q_d}{p_d}}{u-d}$$

This seems to be some replicating portfolio that is expected to payoff 1 at $t=1$.

Question: Any intuition?

Well, $(x,y)=(\frac{1}{1+R},0)$ seems to give the same payoff but with -100% lower risk


For $X\frac{d \mathbb Q}{d \mathbb P}$,

we could say that the price of X uses not

$$E[X] = X_up_u+X_dp_d$$

but rather

$$E^{\mathbb Q}[X] = E[X\frac{d \mathbb Q}{d \mathbb P}] = X_u\frac{q_u}{p_u}p_u + \frac{q_d}{p_d}p_d$$

Question: So what's a '$X\frac{d \mathbb Q}{d \mathbb P}$'?

It pays $X_u\frac{q_u}{p_u}$ or $X_d\frac{q_d}{p_d}$ then replicating portfolio is...then idk. Not sure we need one since we're using real world probabilities anyhoo

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