Symbol f

The passage around this formula

For a static observer at fixed areal radius r in a static, spherically symmetric spacetime, proper time and coordinate time are related by d$\tau$ = $\sqrt{f(r)}$\,dt , with f(r) = 1-2GM/(r$c^2$) for Schwarzschild. A locally conserved (Killing) energy $E_\infty$ — the energy the same quantity would be assigned by a distant static observer — and the energy measured by the local static observer, $E_{\rm loc}$ , are related by the standard blueshift-on-the-way-down relation

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