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It is a useful concept in analysis, indicating lack of an element where one might be expected. It is usually written with the symbol "∅", in Unicode U+2205 ∅ EMPTY SET (∅, ∅, ∅, ∅). A common ad hoc solution is to use the Scandinavian capital letter Ø instead. There are several kinds of zero:
In any topological space X, the empty set is open by definition, as is X. Since the complement of an open set is closed and the empty set and X are complements of each other, the empty set is also closed, making it a clopen set. Moreover, the empty set is compact by the fact that every finite set is compact. The closure of the empty set is empty.
The English language was a fusional language, this means the language makes use of inflectional changes to convey grammatical meanings. Although the inflectional complexity of English has been largely reduced in the course of development, the inflectional endings can be seen in earlier forms of English, such as the Early Modern English (abbreviated as EModE).
The empty set is a subset of every set (the statement that all elements of the empty set are also members of any set A is vacuously true). The set of all subsets of a given set A is called the power set of A and is denoted by or (); the "P" is sometimes in a script font: ℘ .
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Given any set A, there is a set B (a subset of A) such that, given any set x, x is a member of B if and only if x is a member of A and φ holds for x. Note that there is one axiom for every such predicate φ; thus, this is an axiom schema. To understand this axiom schema, note that the set B must be a subset of A.
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Furthermore, one sometimes considers set theories in which there are no infinite sets, and then the axiom of empty set may still be required. However, any axiom of set theory or logic that implies the existence of any set will imply the existence of the empty set, if one has the axiom schema of separation. This is true, since the empty set is a ...