Math 2.0

object

Empty set

Description

The empty set is the set that has no elements. It is written ∅\varnothing or {}\{\}. For each object xx, x∉∅x \notin \varnothing. By the axiom of extensionality, there is only one empty set.

Remarks

The empty set is a subset of every set. This is a case of vacuous truth. Its cardinality is 00. The set {∅}\{\varnothing\} is not empty: it has one element.

Explanations

L1 human

The empty set is an empty bag. For example, the set of real numbers xx with x2=−1x^2 = -1 is empty. It is useful in the same way that the number 00 is useful.

What this node needs for W2
  • section Invariants: a table of the main invariants (0 of 1)

The writing levels say how complete the text of a node is. They do not measure its truth.

What depends on this

149 nodes use this node, directly or through other nodes.

Sources

  1. Web https://en.wikipedia.org/wiki/Empty_set, lead section