Fix , let be Brownian motion, and let . Set for andThe hitting time is finite almost surely, so each price path reaches one before and stays there. Stopping additionally on hitting makes the stock price a bounded martingale; these stopping times increase to . Hence is a positive local martingale, and the original probability measure is an equivalent local martingale measure, excluding admissible arbitrage. Nevertheless while . The zero-cost short-stock/long-two-cash portfolio has terminal gain one but wealth , which has no deterministic lower bound. It is not admissible, resolving the apparent contradiction.
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