Definition
In an isotropic chart (on a static spherically symmetric spacetime), the line element takes the form
Depending on context, it may be appropriate to regard f,g as undetermined functions of the radial coordinate (for example, in deriving an exact static spherically symmetric solution of the Einstein field equation). Alternatively, we can plug in specific functions (possibly depending on some parameters) to obtain an isotropic coordinate chart on a specific Lorentzian spacetime.
Read more about this topic: Isotropic Coordinates
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