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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.
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 ...
Set theory is the branch of mathematical logic that studies sets, which can be informally described as collections of objects.Although objects of any kind can be collected into a set, set theory – as a branch of mathematics – is mostly concerned with those that are relevant to mathematics as a whole.
In any Euclidean space, the interior of any finite set is the empty set. On the set of real numbers, one can put other topologies rather than the standard one: If is the real numbers with the lower limit topology, then ([,]) = [,).
Standard set theory symbols with their usual meanings (is a member of, equals, is a subset of, is a superset of, is a proper superset of, is a proper subset of, union, intersection, empty set) ∧ ∨ → ↔ ¬ ∀ ∃ Standard logical symbols with their usual meanings (and, or, implies, is equivalent to, not, for all, there exists) ≡
A space is a T 1 space if every subset consisting of a single point is closed. [8] In a T 1 space, the derived set of a set consisting of a single element is empty (Example 2 above is not a T 1 space). It follows that in T 1 spaces, the derived set of any finite set is empty and furthermore, ({}) ′ = ′ = ({}) ′, for any subset and any ...
In set theory, an infinite set is not considered to be created by some mathematical process such as "adding one element" that is then carried out "an infinite number of times". Instead, a particular infinite set (such as the set of all natural numbers) is said to already exist, "by fiat", as an assumption or an axiom. Given this infinite set ...
A Sperner family is a set family in which none of the sets contains any of the others. Sperner's theorem bounds the maximum size of a Sperner family. A Helly family is a set family such that any minimal subfamily with empty intersection has bounded size. Helly's theorem states that convex sets in Euclidean spaces of bounded dimension form Helly ...