[19F] From the exercises [172]↺↻ and [174]↺↻ it follows that if \(x∈ {{A}^\circ }\) or \(y∈ {{A}^\circ }\) then \( (tx+(1-t)y)∈ {{A}^\circ }\): so the definition is ”interesting” when \(x,y∈ ∂ A\).
