Symbol phi_,i

The passage around this formula

Define the network’s curvature-diagnosing observable directly from this result, restoring physical units through the relation between coordinate acceleration and proper time for a static observer, $\mathcal T_{ij} := c^2\left(\phi_{,ij} + \phi_{,i}\,\phi_{,j}\right)$. a symmetric rank-two tensor in the local spatial frame of the static observers, with units of inverse time squared — the Eötvös units already standard in gravity-gradient surveying, where 1\,$\mathrm E$ = 10^{-9}\,$\mathrm{s^{-2}}$ . Under a rotation of the local spatial axes it transforms as an ordinary rank-two tensor; under a change of which node is used as the additive reference for $\phi$ it is unchanged, because it depends only on derivatives of $\phi$ , never its value; and up…

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