Then f is convex, and ⟨ ⟨ p , x ⟩ ⟩ − − f ( x ) = ⟨ ⟨ p , x ⟩ ⟩ − − ⟨ ⟨ x , A x ⟩ ⟩ − − c , has gradient p − 2 Ax and Hessian −2 A, which is negative; hence the stationary point x = A p /2 is a maximum.