b (source code)
= b
{scope}
For the three cases, distinguish a <curvature singularity> from a <coordinate singularity>. The <Reissner-Nordstrom Kretschmann scalar> is
$$
\mathcal K=R_{\alpha\beta\gamma\delta}R^{\alpha\beta\gamma\delta}
=\frac{48M^2}{r^6}-\frac{96MQ^2}{r^7}+\frac{56Q^4}{r^8}.
$$
It diverges at $r=0$ when either $M$ or $Q$ is nonzero. Any positive root of $F$ instead admits <Eddington-Finkelstein coordinates>: set $v=t+\int dr/F$ to obtain
$$
ds^2=-F\,dv^2+2\,dv\,dr+r^2(d\theta^2+\sin^2\theta\,d\phi^2).
$$
The <determinant> of its $(v,r)$ block is $-1$, including at $F=0$. Hence positive roots of $F$ are removable <coordinate singularities> of the static chart. The apparent degeneracy at the poles of the angular chart is also a <coordinate singularity>.
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