Probably a really stupid question..(adsbygoogle = window.adsbygoogle || []).push({});

##u=t+r+2M ln(\frac{r}{2M}-1) ##

From this I get

##\frac{du}{dr}=(1-\frac{2M}{r})^{-1}##

But, 1997 Sean M. Carroll lectures notes get ##\frac{du}{dr}=2(1-\frac{2M}{r})^{-1}## . (equation 7.71).

No idea where this factor of 2 comes from.

Thanks

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# Really quick algebra question -- Eddington-Finkelstein coordinates

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