Boundary Region A

RT formula (2006):
S_A = Area(γ_A)/(4G_N)
where γ_A is minimal surface in bulk homologous to A.

CFT result:
S_A = (c/3)log(ℓ/ε) for interval ℓ in CFT₂. Matches with c = 3L/(2G_N).

Phase transition:
For two intervals: minimal surface can "swap" — entanglement phase transition.

FLM (2013):
S = Area/4G_N + S_bulk
quantum corrections from bulk fields.