= Minkowski conformal compactification
{c}
{title2=$\bar g=-dT^2+dR^2+\sin^2R\,d\Omega^2$}
For four-dimensional <Minkowski spacetime>, set $p=\arctan((t-r)/L)$ and $q=\arctan((t+r)/L)$, with $L>0$, then $T=p+q$, $R=q-p$. Multiplication by the square of the <conformal factor> $2\cos p\cos q/L$ yields the displayed metric on $R\geq0$, $|T|+R<\pi$. Its radial <Penrose diagram> is a triangle: the timelike edge $R=0$ is the ordinary centre, while the two sloping edges are <future null infinity> and <past null infinity>. A signed Cartesian spatial coordinate in two dimensions gives a full diamond instead.
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