Past exam of the mathematics course of the University of Cambridge 2016 iii Paper 211 1 b Solution Created 2026-10-03 Updated 2026-10-06
Write . Conditional on , independent increments of the Brownian motion givewhere has the standard normal distribution and is independent of . Factoring out of the positive part identifies the normalized Black-Scholes call function:The European call option payoff is integrable, because and . The tower property of conditional expectation therefore proves that is a true martingale. At , the boundary value gives . The answer is the conditional payoff martingale