Equality (mathematics)

In mathematics, two mathematical objects are considered equal if they are precisely the same in every way. This defines a binary predicate, equality, denoted "="; x = y iff x and y are equal. Equivalence in the general sense is provided by the construction of a equivalence relation between two elements. A statement that two expressions denote equal quantities is an equation.

Beware that sometimes a statement of the form "A = B" may not be an equality. For example, the statement T(n) = O(n²) means that T(n) grows at the order of n². It is not an equality, because the sign "=" in the statement is not the equality sign; indeed, it is meaningless to write O(n²) = T(n). See Big O notation for more on this.

Given a set A, the restriction of equality to the set A is a binary relation, which is at once reflexive, symmetric, antisymmetric, and transitive. Indeed it is the only relation on A with all these properties. Dropping the requirement of antisymmetry yields the notion of equivalence relation. Conversely, given any equivalence relation R, we can form the quotient set /R/R. Note that it may be impractical to compute with equivalence classes: one solution often used is to look for a distinguished normal form representative of a class.

Predicate logic contains standard axioms for equality that formalise Leibniz's law, put forward by the philosopher Gottfried Leibniz in the 1600s. Leibniz's idea was that two things are identical if and only if they have precisely the same properties. To formalise this, we wish to say

Given any x and y, x = y if and only if, given any predicate P, P(x) iff P(y).