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 The Gautreau-Hoffmann coordinates are very similar to the coordinates of Sec. III. They also constitute a oneparameter family of coordinate systems, and their parameter Ri is related to our p by Ri = 2M p/(p − 1). The difference lies with the fact that while their Ri is not meant to be negative, our p is restricted to the interval 0 < p ≤ 1. These are mutually exclusive statements. But the point remains that formally, the coordinates of Sec. III are identical to the Gautreau-Hoffmann coordinates for Ri < 0. Note that for Ri > 0 (the case considered by Gautreau and Hoffmann in Ref. ), the surfaces of constant time extend only up to r = Ri; they do not reach infinity.
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 This strategy was used by I.D. Novikov to construct yet another set of coordinates for Schwarzschild spacetime. The reference is I.D. Novikov, Doctoral dissertation, Shternberg Astronomical Institute, Moscow (1963). The Novikov coordinates are discussed in Sec. 31.4 of Ref. . Unlike the coordinates considered in this paper, the Novikov coordinates are comoving with respect to the observers to which they are attached. This means that these observers move with a constant value of Novikov's spatial coordinates.
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 For p > 1, or E˜ < 1, the motion does not extend to in-