Poset Definition Given a poset A, < and a set SCa u∈ A is a upper bound of s if s≤ u for every s∈S. @lE A is a lower bound of s if l≤ s for every s∈S.Poset Definition Given a poset < A, ≤> and a set S ⊆ A. 1 u ∈ A is a upper bound of S if s ≤ u for every s ∈ S. 2 l ∈ A is a lower bound of S if l ≤ s for every s ∈ S. Yi Li (Fudan University) Discrete Mathematics February 28, 2012 6 / 15