Tutorial Contents Tutorial Eight: Relations- Reflexivity - Symmetry -Transitivity - Connectedness -
Example of a tableau proof
- Eqivalent Relations
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8: Relations


Binary relations


A binary relation is the sort of relation which can be expressed by a 2-place predicate: for example:


x loves y


x is greater than y


[x is a cow y is a blade of grass]

Hodges uses a convenient way of representing a relation. Consider a relation expressed by "Rxy". Then we represent the domain by a circle, and the individuals in the domain by dots labelled, perhaps, "a", "b", "c", and so on; we represent the fact that Rab, say, by drawing an arrow from dot a to dot b; we represent the fact that both Rab and Rba by drawing a double arrow between a and b; and we represent the fact that Raa by drawing a double arrow looping from a back to itself (a "loop", for short). The whole representation of the relation is its graph.


This way of thinking of relations provides a useful way of describing various properties which a relation may or may not have.


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