Christoffel symbols in Kruskal-Szekeres coordinates What are the Christoffel symbols for the Schwarzschild spacetime, expressed in the Kruskal-Szekeres coordinates?
 A: I worked these out recently, and it was a fair amount of computation to get them, so I thought they might be useful to others.
Throughout the following, let $m=1/2$, so the horizon is at $r=1$, i.e., $r$ is in units of the Schwarzschild radius. The exterior regions of the maximal extension of the Schwarzschild spacetime are at $r>1$, which is regions I and III. The interior is $r<1$, regions II and IV.
The version of the Kruskal-Szekeres coordinates I'll use are null coordinates $(V,U)$, equivalent to Hawking and Ellis's $(v'/\sqrt2,w'/\sqrt2)$.
Even when working in the Kruskal-Szekeres coordinates, it's convenient to express some things in terms of the Schwarzschild $r$ coordinate, which can be found using
\begin{equation}
  r = 1+W(-VU/e).
\end{equation}
Here the function $W$ is the principal real branch of the Lambert W function, and $e$ is the base of natural logarithms.
It's also convenient to define
\begin{equation}
  B = \frac{4}{re^r}.
\end{equation}
The metric is
\begin{equation}
  ds^2 = B dVdU-r^2 d\Omega^2.
\end{equation}
The Christoffel symbols are as follows:
\begin{align}
\Gamma^V_{VV} &= (r^{-1}+r^{-2})Ue^{-r}  \\
\Gamma^U_{UU} &= (r^{-1}+r^{-2})Ve^{-r}  \\
\Gamma^\theta_{V\theta} = \Gamma^\phi_{V\phi} &=  -UB/4r \\
\Gamma^\theta_{U\theta} = \Gamma^\phi_{U\phi} &= -UB/4r \\
%
\Gamma^V_{\theta\theta} &= -Vr/2 \\
\Gamma^U_{\theta\theta} &= -Ur/2 \\
\Gamma^V_{\phi\phi} &= -(Vr/2) \sin^2\theta  \\
\Gamma^U_{\phi\phi} &=  -(Ur/2) \sin^2\theta \\
%
\Gamma^\theta_{\phi\phi} &= -\sin\theta \cos\theta   \\
\Gamma^\phi_{\theta\phi} &= \cot\theta   \\
\end{align}
I got these by calculating them in the computer algebra system Maxima and then cleaning up the resulting expressions by hand. I checked my cleaned-up versions numerically against the raw output from Maxima to make sure they were right. They are implemented in an open-source software project called karl.
The metric and the Christoffel symbols misbehave at the $r=0$ singularities, and also at the coordinate singularities at $\theta=0$ and $\pi$.
Related: Free-fall path into a black hole in Kruskal Coordinates
