How can probabilities that don't add up to one hand a counterparty a riskless profit?
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Incoherent beliefs and the theorem
Write the event A, an S&P 500 fall of 5% or more next Monday, with and , and state the theorem for probabilities not summing to one.
Constructing the Dutch book
Set out bets and at the odds those beliefs make fair, stake $50 on and $25 on , and tabulate the $25 profit in each state.
The efficient-markets retort and its limit
Record the claim that smart money arbitrages mispricings until beliefs cohere, the fixed-income arbitrage example, and the warning that markets can stay irrational longer than an arbitrageur stays solvent.