The Ding--Song--Sun inequality via a maximum principle

We prove the Ding--Song--Sun inequality for continuous spin models on finite ferromagnetic graphs whose even single-site potentials have convex derivatives on the positive half of their domain. This class, introduced by Ellis, Monroe and Newman, includes the $\varphi^4$ and sinh-Gordon potentials. The proof uses a rank-one interpolation of the interactions and a maximum principle for a cooperative parabolic system on a half-plane. The boundary conditions follow by induction on the number of vertices and the number of positive coordinates of the field increment. In particular, the truncated two-point function in an arbitrary external field is bounded above by its zero-field value.