In mathematics, equality is a relationship between two quantities or, more generally two mathematical expressions, asserting that the quantities have the same value, or that the expressions represent the same mathematical object. The equality between and is written, and pronounced equals. The symbol "" is called an "equals sign". Two objects that are not equal are said to be distinct. For example:
means that and denote the same object.
means that, if is any number, the two expressions have the same value. This may also be interpreted by saying that the two sides of the equals sign represent the same function.
if and only if This assertion, which uses set-builder notation, means that, if the elements satisfying the property are the same as the elements satisfying then the two uses of the set-builder notation define the same set. This property often expressed as "two sets that have the same elements are equal." It is one of the usual axioms of set theory, called Axiom of extensionality.
When A and B are not fully specified or depend on some variables, equality is a proposition, which may be true for some values and false for some other values. Equality is a binary relation or, in other words, a two-argument predicate, which may produce a truth value from its arguments. In computer programming, its computation from two expressions is known as comparison.
Identities
When A and B may be viewed as functions of some variables, then A = B means that A and B define the same function. Such an equality of functions is sometimes called an identity. An example is 2 = x2 + 2x + 1. Sometimes, but not always, an identity is written with a triple bar: 2 ≡ x2 + 2x + 1.
Equations
An equation is a problem of finding values of some variables, called unknowns, for which the specified equality is true. Equation may also refer to an equality relation that is satisfied only for the values of the variables that one is interested in. For example, x2 + y2 = 1 is the equation of the unit circle. There is no standard notation that distinguishes an equation from an identity or other use of the equality relation: a reader has to guess an appropriate interpretation from the semantics of expressions and the context. An identity is asserted to be true for all values of variables in a given domain. An "equation" may sometimes mean an identity, but more often it specifies a subset of the variable space to be the subset where the equation is true.
Congruences
In some cases, one may consider as equal two mathematical objects that are only equivalent for the properties that are considered. Particularly in the case of geometry, this is where two geometric shapes are said to be equal when one may be moved to coincide with the other. The word congruence is also used for this kind of equality.
Approximate equality
There are some logic systems that do not have any notion of equality. This reflects the undecidability of the equality of two real numbers defined by formulas involving the integers, the basic arithmetic operations, the logarithm and the exponential function. In other words, there cannot exist any algorithm for deciding such an equality. The binary relation "is approximately equal" between real numbers or other things, even if more precisely defined, is not transitive. However, equality almost everywhereis transitive.
Viewed as a relation, equality is the archetype of the more general concept of an equivalence relation on a set: those binary relations that are reflexive, symmetric and transitive. The identity relation is an equivalence relation. Conversely, let R be an equivalence relation, and let us denote by xR the equivalence class of x, consisting of all elements z such that x R z. Then the relation x R y is equivalent with the equality xR = yR. It follows that equality is the finest equivalence relation on any set S in the sense that it is the relation that has the smallest equivalence classes. In some contexts, equality is sharply distinguished from equivalence or isomorphism. For example, one may distinguish fractions from rational numbers, the latter being equivalence classes of fractions: the fractions and are distinct as fractions but they "represent" the same rational number. This distinction gives rise to the notion of a quotient set. Similarly, the sets are not equal sets — the first consists of letters, while the second consists of numbers — but they are both sets of three elements and thus isomorphic, meaning that there is a bijection between them. For example However, there are other choices of isomorphism, such as and these sets cannot be identified without making such a choice — any statement that identifies them "depends on choice of identification". This distinction, between equality and isomorphism, is of fundamental importance in category theory and is one motivation for the development of category theory.
Equality of sets is axiomatized in set theory in two different ways, depending on whether the axioms are based on a first-order language with or without equality.
Incorporating half of the work into the first-order logic may be regarded as a mere matter of convenience, as noted by Lévy.
Set equality based on first-order logic without equality
In first-order logic without equality, two sets are defined to be equal if they contain the same elements. Then the axiom of extensionality states that two equal sets are contained in the same sets.
