R1: standard Schwarzschild to ingoing Eddington–Finkelstein coordinates, all real $M$

Let $M$ be an arbitrary real number. With $g^{\rm std}_M$ the standard Schwarzschild metric $g^{\rm std}_M$ (bundle Atlas_Schwarzschild_Defs, coordinates $(t,r,\theta,\varphi)$): $g_{tt}=-(1-2M/r)$, $g_{rr}=1/(1-2M/r)$, $g_{\theta\theta}=r^2$, $g_{\varphi\varphi}=r^2\sin^2\theta$, other entries $0$, with candidate inverse $\hat g^{tt}=-1/(1-2M/r)$, $\hat g^{rr}=1-2M/r$, $\hat g^{\theta\theta}=1/r^2$, $\hat g^{\varphi\varphi}=1/(r^2\sin^2\theta)$, $g^{\rm EF}_{M,1}$ the Eddington–Finkelstein family $g^{\rm EF}_{M,\varepsilon}$ (bundle Atlas_Schwarzschild_Defs, coordinates $(w,r,\theta,\varphi)$ with $w=v$ for $\varepsilon=1$ ingoing and $w=u$ for $\varepsilon=-1$ outgoing): $g_{ww}=-(1-2M/r)$, $g_{wr}=g_{rw}=\varepsilon$, $g_{\theta\theta}=r^2$, $g_{\varphi\varphi}=r^2\sin^2\theta$, other entries $0$, with candidate inverse $\hat g^{wr}=\hat g^{rw}=\varepsilon$, $\hat g^{rr}=1-2M/r$, $\hat g^{\theta\theta}=1/r^2$, $\hat g^{\varphi\varphi}=1/(r^2\sin^2\theta)$, other entries $0$ at $\varepsilon=1$, the map $\Phi(t,r,\theta,\varphi)=\big(t+r+2M\log\lvert r-2M\rvert,\ r,\ \theta,\ \varphi\big)$, its inverse $\Psi(v,r,\theta,\varphi)=\big(v-r-2M\log\lvert r-2M\rvert,\ r,\ \theta,\ \varphi\big)$, the Jacobian $J$ with $J_{01}=1+\frac{2M}{r-2M}$, $J_{ii}=1$ and all other entries $0$, and the overlap $U^{\rm std}_M=\{x:\ r>0,\ r\neq2M,\ 0<\theta<\pi\}$ ($t,\varphi$ arbitrary; for $M>0$ this has an exterior and an interior component):

$\mathrm{CoordRelation}(g_1,g_2,U,V,\Phi,\Psi,J)$ (bundle Atlas_Core_Relation) asserts: $\Psi(\Phi(x))=x$ for $x\in U$; $\Phi(\Psi(y))=y$ for $y\in V$; $\Phi$ maps $U$ bijectively onto $V$; $\Phi$ is $C^\infty$ on $U$ and $\Psi$ is $C^\infty$ on $V$; for every $x\in U$ and all indices $k,i$ the slice $u\mapsto\Phi(x[x_i:=u])_k$ has derivative $J(x)_{ki}$ at $u=x_i$ (a genuine derivative, not a default value); and $J(x)^{\mathsf T}\,g_2(\Phi(x))\,J(x)=g_1(x)$ for all $x\in U$.

Relation R1 of the atlas: the explicit change of coordinates between rows 3 and 4 on the whole standard domain (exterior and, for $M>0$, interior). Statements at $r=2M$ are not derived from this relation but from the direct EF certificate on the larger domain.

Formalization Note Everything is coordinate-level on $\mathbb R^4$ (points are functions $\{0,1,2,3\}\to\mathbb R$): no manifold, chart gluing, periodicity of $\varphi$, horizon, causal or global statement is made. Partial derivatives are Mathlib's deriv of coordinate slices; the certificates' $C^\infty$ obligations make every derivative used a genuine derivative. Units $G=c=1$, signature $(-,+,+,+)$.

Formalization Note (logarithm). $\log\lvert r-2M\rvert$ stands for Mathlib's Real.log (r - 2M), which equals $\log\lvert r-2M\rvert$ for every $r\neq2M$ (Real.log_abs); the Lean expression is unchanged and the domain is not restricted to $r>2M$.