We shall use the symbol (known as tilde or twiddle) to represent a relation between two elements of a set.
Some texts use ρ, rather than , for an arbitrary relation. Certain relations have special symbols; for example,
< means is less than,
= means is equal to.
We say that is a relation on a set X if, whenever x, y X, the statement x y is either true or false.
If x y is true, then x is related to y.
If x y is false, then x is not related to y and we write x y.
The statement x y can be read as ‘x is related to y’ or ‘x twiddles y’.