In mathematics, the symmetric closure of a binary relation R on a set X is the smallest symmetric relation S on X that contains R. Restated:
- S is symmetric
- R ⊆ S
- For any relation S’, if R ⊆ S’ and S’ is symmetric then S ⊆ S’. In other words, S is smallest relation that satisfies 1 and 2.
The symmetric closure S of a binary relation R on a set X can be formally defined as:
S = R ∪ {(x, y) : (y, x) ∈ R}
Where {(x, y) : (y, x) ∈ R} is the inverse relation of R, R-1. The symmetric closure of a binary relation on a set is the union of the binary relation and it’s inverse.
For example, let R be the greater than relation on the set of integers I:
R = {(a, b) | a ∈ I ∧ b ∈ I ∧ a > b}
The symmetric closure is the union of the relation (greater than) and its inverse (less than) over I. The union of greater than and less than is not equal to, ≥ or ≠. So the symmetric closure S of the greater than relation R is:
S = {(a, b) | a ∈ I ∧ b ∈ I ∧ a <> b} = {(a, b) | a ∈ I ∧ b ∈ I ∧ a ≠ b}
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