Mathematical Proof and the Principles of Mathematics/Sets/Union and intersection

Unions of setsEdit

The construction that allows us to form sets with more than two elements is the union. It allows us to take existing sets and form a single set containing all the elements of those sets.

Axiom (Union)

Given a set   of sets, there exists a set   such that   if and only if   for some  .

Definition Given a set   of sets, we call a set   as in the Axiom of Union, a union over   and denote it  .

Example Let  ,   and  . Now let  . Then  .  

Theorem Given a set   of sets as in the Axiom of Union, the union over   is unique.

Proof If   and   were both unions over   then   iff   for some  . Similarly   iff   for some  . Thus   iff  , and so   by extensionality.  

We recover the familiar definition of the union of two sets as follows.

Definition If   we denote   by   and call it the union of   and  .

Theorem If   and   are sets then   is a set.

Proof By the Axiom of Pair,   is a set. Thus by the Axiom of Union   is a set.  


Comprehensions allow us to select elements of an existing set that have some specified property. The Axiom Schema of Comprehension says that such selections define sets.

There is very little restriction on the properties we may use in comprehensions, except that they must be specified with formulas in the language of set theory and formal logic.

We first define what we mean by a formula.

Definition A formula can contain variables   of which we are allowed an unlimited supply, and constants, i.e. specific sets   say, and must be built up using a finite number of the following:

  • Expressions of the form   and   are formulas, called atomic formulas, for all variables and constants   and  .
  • If   and   are formulas then  ,  ,  ,   and   are formulas.
  • If   is a formula then   and   are formulas.

Here   stands for logical or,   is logical and,   is logical negation,   is implies and   stands for if and only if, which we shall often abbreviate iff.

The expression   is called a universal quantifier. It means for all sets  . The expression   is an existential quantifier. It means there exists a set  .

Example Given a set   the expression   is an example of a formula.  

Although symbols  ,  , etc., and   for there exists a unique, are not part of the formal language, we can define them in terms of the existing language of set theory.

For example,   can be written  . Similarly,   can be written  .

Definition In a formula, any variable   inside an expression of the form   or of the form   is said to be bound. All other variables in a formula are said to be free, or arguments of the formula.

We are now in a position to state the Axiom Schema of Comprehension.

Axiom Schema (Comprehension)

For a set   and a property   there exists a set   consisting of the   for which   holds.

Note that this is not just one axiom, but an axiom for each possible property. We call a collection of axioms like this an axiom schema.

Technically the formula   can have finitely many free variables, so is sometimes denoted   where the   are free variables. But we suppress this technicality for now and just write  .

Theorem The set of elements of a set   for which a property   holds is unique.

Proof This follows by noting that if there were two such sets  and   then   iff   and   holds. However, this is the case iff  . Thus   iff   and the result follows by extensionality.  

Definition The set of elements of a set   for which   holds is denoted  .

We can read the vertical bar as such that.

Example If   then  .

The Axiom Schema of Comprehension is sometimes called the Subset Axiom Scheme or Axiom Schema of Specification, since it guarantees that any subset of a set specified by a formula, is a set.

Intersections of setsEdit

We can define the familiar intersection of two sets in terms of a comprehension.


The formula in the comprehension consists of two predicates from the language of sets,   and  , joined by the logical conjunction and from formal logic.

More generally, we have the following.

Definition Let   be a set of sets. The intersection over   is defined by


Example Let  ,   and  . If   then  .

Theorem Let   be a set of sets. Then   is a set.

Proof This is a set by the axioms of union and comprehension.  

The following is a useful definition.

Definition Two sets   and   are said to be disjoint if  .

Example The sets   and   are disjoint since their intersection is empty.


  • Show that if   and   are sets then   if and only if  .
  • Let  ,   and   be sets. Show that there exists a set whose elements are  ,   and  .
  • Suppose   and   are sets with   for all  . Let   and  . Show that  .

Pairs · Classes and foundation