= Fréchet-Bell inequality
{c}
{title2=$P(A\setminus C)\leq P(A\setminus B)+P(B\setminus C)$}
For three events in one classical <probability space>, the inclusion $A\setminus C\subset(A\setminus B)\cup(B\setminus C)$ and the <union bound> prove the displayed inequality. It is therefore an unconditional theorem of classical <probability>. In a polarization Bell test, a model assigning simultaneous setting-independent pass/fail outcomes to three possible analyzer directions must obey it. Equal unbiased marginals and quantum pair correlations instead give $P(A\setminus B)=\tfrac12\sin^2(\theta_A-\theta_B)$. Directions $0,\gamma,2\gamma$ violate the resulting constraint for $0<\gamma<\pi/4$, because $2\sin^2\gamma<\sin^22\gamma$. The conclusion excludes that common classical joint model. Conditional probabilities from sequential measurements cannot be inserted without their marginal factors or treated as if all three contexts shared an already established joint distribution.
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