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This paper gives two definitions of Submodular Function. Let $N$ be the base set, and let $\mathcal P(N)$ denote the powerset of $N$.

Definition 1: A function $z: \mathcal P(N) \to \mathbb R^+$ is a nondecreasing submodular function if $z(A) + z(B) \ge z(A \cup B) + z(A \cap B)$.

Now, for $j \in N$ and $S \subset N$, define $\rho_j(S) = z(S \cup \{j\}) - z(S)$

Definition 2: A function $z: \mathcal P(N) \to \mathbb R^+$ is submodular and nondecreasing iff either

  1. $\rho_j(S) \ge \rho_j(T) \ge 0$ $\qquad \forall S \subseteq T \subseteq N\qquad $ or
  2. $z(T) \le z(S) + \sum_{j \in T \setminus S} \rho_j(S)$ $\qquad \forall S, T \subseteq N$

How do I prove that the two definitions are equivalent? I have tried plugging in different types of sets (like $A, B$ such that $A \subseteq B$) in both definitions, but no luck.

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You can find a solution to this on Page Number 420-421 of the following book on Submodular Functions. here

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