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A relation between a domain and its codomain is an Equivalence relation if and only if it is reflexive, symmetric and transitive.

A = { (x,y) | x is a friend of y }

For example:

D = { a , b, c, d }

A : D --> D

= { (a, a) , (a, b) , (b, a) , (c, d), (d, c), (b, b), c, c), (d, d) }

A = { (x,y) | x is a friend of y }

For example:

D = { a , b, c, d }

A : D --> D

= { (a, a) , (a, b) , (b, a) , (c, d), (d, c), (b, b), c, c), (d, d) }