Symbol c^2

The passage around this formula

…reference for $\phi$ it is unchanged, because it depends only on derivatives of $\phi$ , never its value; and up to the factor $U^2$ $\approx$ 1 negligible at the one-part-in- 10^9 level for any terrestrial network, it is $c^2$ times the frame component $R^i${}_{0j0} of the Riemann tensor in the static observers’ own rest frame — a genuine curvature quantity, reported honestly as one stated-frame component rather than a coordinate-free scalar invariant, exactly the caveat this construction owes a reader.

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