How come theres no theory that attempts to classify spacetime functions under some change of coordinates, like rotations or Lorentz transformations. From a mathematical standpoint this field seems very clunky, like "eh try out this function and see of it doesnt change".
Sometimes you can come up with easy solutions for the obvious case, like any function that only depends on a spatial radius will be invariant against rotations, i have not see anything so simple with other types of transformations.