and we observe that the ratios μ−r
σm
and (β(Mt )−1)r
σ are bounded. By Girsanov theorem,
there is a unique equivalent probability P ∼ P such that (W∗
t , Z∗
t ) are independent
standard Brownian motions under P, called the pricing equivalent martingale
measure or risk-neutral measure. Under P, the dynamics (6, 7) becomes: