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The passage around this formula

the proper spatial separation within $\Sigma$ between the two charts’ answers, normalized by the spin-to-mass length fixed in the first section. $\delta_{ij}$ is dimensionless by construction, symmetric, zero exactly when the two charts coincide, and it has now been checked, not merely asserted, in both directions this kind of construction is required to pass: it vanishes as v/c $\to$ 0 for any spin, and it vanishes as S $\to$ 0 for any observer or Lorentz frame. It is bounded above by exactly 1 for the observer-transition case, since |$\Delta$$\mathbf x$| $\le$ $\rho_M$ was shown above to saturate rather than merely approach that value. Both checks are this construction’s kill criterion, run rather than…

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