Solution

ID: past-exam-of-the-martingales-and-financial-mathematics-course-of-the-university-of-oxford/2011/b/6/c/solution

For an ex-dividend stock, holding shares earns both and the dividend gain . Freeze locally in a self-financing hedge that is long the claim and short these shares. Itô's formula cancels its Brownian exposure, leaving drift . Its value is , so absence of arbitrage requires this drift to equal . Rearrangement gives the Black-Scholes equation with continuous stock dividends:
With the given parameters,
The physical drift cancels because the hedge removes the stock-price risk; the dividend payment remains in the stock's total gain.

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