# Typo in Sagemanifolds worksheet ? [closed] Sorry to bother the group but I don't know how to report a technical (but inconsequential) typo in Kerr_Schild.ipynb Where should I report it? After In or so, "Check of the Identity"
 \frac{x^2 + y^2}{r^2 + a^2} + \frac{z^2}{r^2} = 1 
Should be:
 \frac{x^2 + y^2}{R^2 + a^2} + \frac{z^2}{R^2} = 1 
It seems to be too trivial for a 'Ticket' on the Sagemanifolds forum. I am just reading the program in detail; checking my understanding.

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### Closed for the following reason the question is answered, right answer was accepted by rrogers close date 2020-09-07 14:39:20.270197

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Even if it were a mistake (which is not, see the answer below), it would not have been worth a ticket, because this is not an issue with Sage code nor standard documentation. It regards only a comment in some Markdown cell of this Jupyter notebook.

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This is not a typo: $R$ is actually equal to $r$, see e.g. Sec. C.2.2 in Appendix C of these lecture notes, in particular compare Eq. (C.22) to Out of the notebook. The notation $R$ is used to distinguish the function of $(x,y,z)$ from the coordinate $r$, but numerically, we have $r = R(x,y,z)$.

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