Past exam of the mathematics course of the University of Cambridge / 2019 / ii / Paper 2 / 29K / a / Solution

An equivalent martingale measure is a probability measure $\mathbb{Q}$ on the terminal sigma-algebra such that $\mathbb{Q}$ and the physical measure $\mathbb{P}$ are mutually absolutely continuous and every asset price discounted by the chosen numéraire is a $\mathbb{Q}$-martingale with respect to the market filtration.