原文
Definition 1 · DR-submodular function
2.1 Notations and Definitions
Given a convex set 𝒟 ⊆ ℝᵈ, a continuous function f : ℝᵈ → ℝ is said to be DR-submodular over 𝒟, if for any x ≤ y ∈ 𝒟 and a ∈ ℝ₊ …
f(x + aeᵢ) − f(x) ≥ f(y + aeᵢ) − f(y)
and x ≤ y means xᵢ ≤ yᵢ, ∀i ∈ [d].