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Relations

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An 
n
-ary relation is a set 
S
together with a set 
ρS
of ordered tuples 
(s1, ..., sn ) ∈ Sn
.

Binary relations

A binary relation or dyadic relation is a set 
S
together with a set 
ρS
of ordered pairs 
(s, s′ ) ∈ S  ×  S
, i.e. 
{(s, s′ ) ∈ S  ×  S | s ρS s′}
where 
s ρS s:= (s, s′ ) ∈ ρS
.

For example, the set 
S = {1, 2, 3}
together with the set 
"<S" := {(1, 2), (2, 3), (1, 3)}
defines the binary relation 
{(s, s′ ) ∈ S  ×  S | s <S s′}
where 
s <S s:= (s, s′ ) ∈ "<S"
.

For example, the set 
S = {1, 2, 3, 4, 5, 6}
together with the set 
"dividesS" := {(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (1, 6), (2, 2), (2, 4), (2, 6), (3, 3), (3, 6), (6, 6)}
defines the binary relation 
{(s, s′ ) ∈ S  ×  S | s "dividesS" s′}
where 
s "dividesS" s:= (s, s′ ) ∈ "dividesS"
.

Ternary relations

A ternary relation or triadic relation is a set 
S
together with a set 
ρS
of ordered triples 
(s1, s2, s3 ) ∈ S 3
.

For example, the set 
S = {1, 2, 3, 4}
together with the set 
"betweenS" := {(1, 2, 3), (1, 2, 4), (1, 3, 4), (2, 3, 4)}
defines the ternary relation 
{(s1, s2, s3 ) ∈ S 3 | s1 <S s2 <S s3}
where 
s1 <S s2 <S s3 := (s1, s2, s3 ) ∈ "betweenS"
.

External links