¶Intersection of s₁ ≤ and s₂ ≤
The intersection of two lower bounds is a lower bound of the join (or union) of the two types.
(s₁ ≤) ∧ (s₂ ≤) = (s₁ ∪ s₂) ≤
¶Equationally
⟦s₁ ≤⟧ = {x | s₁ ⊆ x} ⟦s₁ ≤⟧ = {x | s₂ ⊆ x} ⟦(s₁ ≤) ∧ (s₂ ≤)⟧ = {x | s₁ ⊆ x} ∩ {x | s₂ ⊆ x} = {x | s₁ ⊆ x ∧ s₂ ⊆ x} = {x | (s₁ ∪ s₂) ⊆ x} = ⟦(s₁ ∪ s₂) ≤⟧
¶Graphically
If s₁ = {0} and s₂ = {2}, then the initial constraints are
s₁ ≤ ┏━━━━━━━━━┓ 🮣──╼┫ {0,1,2} ┣╾──🮢 🮣🮠 ┗━━━━┳━━━━┛ 🮡🮢 ┏━━━┷━━━┓ ┏━━━┷━━━┓ ╵ ┃ {0,1} ┃ ┃ {0,2} ┃ 🮣╸{1,2} ┗━━━┳━━━┫ ┣━━━━━━━┫🮣🮠 ╻ ┏━━┷━━┓🮡🮦🮠 🮥🮤 ╵ ┃ {0} ┠─🮧──╴{1}╶─🮠🮡──╴{2} ┗━━┳━━┛ ╻ ╻ 🮡🮢 ╵ 🮣🮠 🮡──────╴{ }╶──────🮠
and
s₂ ≤ ┏━━━━━━━━━┓ 🮣──╼┫ {0,1,2} ┣╾──🮢 🮣🮠 ┗━━━━┳━━━━┛ 🮡🮢 ╵ ┏━━━┷━━━┓ ┏━━━┷━━━┓ {0,1}╺🮢 ┃ {0,2} ┃ ┃ {1,2} ┃ ╻ 🮡🮢┣━━━━━━━┫ ┣━━━┳━━━┛ ╵ 🮥🮤 🮡🮦🮠┏━━┷━━┓ {0}╶──🮠🮡─╴{1}╶──🮧─┨ {2} ┃ ╻ ╻ ┗━━┳━━┛ 🮡🮢 ╵ 🮣🮠 🮡──────╴{ }╶──────🮠
The intersection should be
(s₁ ≤) ∧ (s₂ ≤) ┏━━━━━━━━━┓ 🮣──╼┫ {0,1,2} ┣╾──🮢 🮣🮠 ┗━━━━┳━━━━┛ 🮡🮢 ╵ ┏━━━┷━━━┓ ╵ {0,1}╺🮢 ┃ {0,2} ┃ 🮣╸{1,2} ╻ 🮡🮢┣━━━━━━━┫🮣🮠 ╻ ╵ 🮥🮤 🮥🮤 ╵ {0}╶──🮠🮡─╴{1}╶─🮠🮡──╴{2} ╻ ╻ ╻ 🮡🮢 ╵ 🮣🮠 🮡──────╴{ }╶──────🮠
Since the join (union) of s₁ and s₂ is {0,2}, we can write this as
(s₁ ≤) ∧ (s₂ ≤) = (s₁ ∪ s₂) ≤