FRET distance dependence: E(R_0) = 1/2; E(0) = 1; E(∞) = 0 (three exact sixth-power witnesses)

FRET-efficiency evaluations: E(r = R_0) = 1/2 exactly (Förster-radius definition); E(r = 0) = 1 exactly (contact limit); E(r → ∞) = 0 exactly (no-coupling limit). Canonical pins: E symbolic form; three exact limit values.