Solution (source code)

= Solution

Across $A|BC$, the <GHZ state> has the Schmidt decomposition
$$
\frac1{\sqrt2}\bigl(|0\rangle_A|00\rangle_{BC}
+|1\rangle_A|11\rangle_{BC}\bigr),
$$
of rank two. Symmetry gives the same rank-two decomposition across $B|AC$ and $C|AB$, so the state is entangled across every bipartition.