The whole picture of martingale approach to asset pricing
The first aim is to find a self-financing portfolio strategy, i.e. F-previsible process, (φt, ψt) for (St, Bt) which replicates the claim X. We can prove by Ito calculus that the strategy inherently is also self-financing for discounted price process (Zt, 1). The existence of such a strategy is proposed by martingale representation theorem, to implement which we have to make the discounted price process a martingale to begin with. This is where the Cameron-Marton-Giranov theorem comes into play. The theory asserts that there exists a Q measure under which Zt, after transformation, becomes an exponential brownian motion which is a Q-martingale. The drift term γt that transforms the brownian motion component of Zt under P measure to that under Q is (μt-rt)/σt, the market price of risk. According to martingale representation theorem, the expectation of the discounted claim under Q measure can be replicated by a self-financing portfolio strategy. The law of one price makes the expectation the present value of the claim.