I have been reading this famous paper of Duffie & Lando (2001) and I have a question regarding how they calculate the price of a bond (the reader of this post will not have to dive deep into the details, but it would certainly be helpful to be familiar with the paper). In particular, they fix a probability space on which an asset $V_t$ follows geometric Brownian motion with drift $\mu \equiv m + \sigma^2/2$ and volatility $\sigma$ and compute the price of their bond (under a filtration which is a strict subset of the filtration of $V$) without changing to the risk neutral measure (see equation (29) and equation (26), specifically). Why is this valid, or am I just missing something entirely?
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In the model setup, they state that all agents are risk-neutral, which would mean $\mathbb{P}=\mathbb{Q}$.
