In set theory, the intersection of two sets is a set containing all elements that are members of both sets. The symbol, ∩, is used to indicate a set intersection.
For example, suppose that Γ and Δ are sets of formulae, as defined as:
The intersection Γ ∩ Δ is therefore the following set:
The intersection of two sets can be defined as follows:
Which means, for any x that is an element of Γ ∩ Δ, x is an element of Γ, and x is a member of Δ.