= Faddeev-Zamolodchikov associativity constraint
{c}
Starting with three ordered <rapidities>, the adjacent-exchange sequences $121$ and $212$ must give identical coefficients for every final species word in the <Faddeev-Zamolodchikov algebra>. The exchange parameters are $\theta$, $\theta+\theta'$ and $\theta'$ because rapidity differences add. These coefficient identities express the spectral <Yang-Baxter equation>. One scalar identity is necessary but generally does not exhaust all the constraints.
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