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Can someone explain to me how (8.21) follows from (8.20). The Picture comes from A first course in general relativity (Schutz). Thanks and regards, Jens Wagemaker enter image description here

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closed as off-topic by Danu, ACuriousMind, Gert, Martin, Daniel Griscom Jan 12 '16 at 18:27

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    $\begingroup$ Hint: The second thing is supposed to be the inverse, but only to first order in $\xi$: Check whether this holds or not! $\endgroup$ – Danu Jan 12 '16 at 13:35
  • $\begingroup$ @Danu I have checked what u have said, and indeed the second thing is the inverse if seconds order terms are neglected, thanks! $\endgroup$ – Jens Wagemaker Jan 12 '16 at 14:01
  • $\begingroup$ This question is solved, but I don't know were to mark it solved. $\endgroup$ – Jens Wagemaker Jan 12 '16 at 14:05
  • $\begingroup$ There is no such mark without an answer posted. $\endgroup$ – Kyle Kanos Jan 12 '16 at 14:18
  • $\begingroup$ Should I then delete the post? $\endgroup$ – Jens Wagemaker Jan 12 '16 at 14:28
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(8.21) is the inverse of (8.20) up to first order.

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