Math 2.0

definition

Ordered pair

Definition

An ordered pair (a,b)(a, b) has a first entry aa and a second entry bb, with this property: (a,b)=(c,d)  ⟺  a=c and b=d.(a, b) = (c, d) \iff a = c \text{ and } b = d. In set theory, the Kuratowski definition is (a,b)={{a},{a,b}}(a, b) = \{\{a\}, \{a, b\}\}. It has the property above.

Remarks

In general (a,b)≠(b,a)(a, b) \neq (b, a), but the sets {a,b}\{a, b\} and {b,a}\{b, a\} are equal. Ordered pairs are the elements of a Cartesian product. An ordered triple can be defined as (a,b,c)=((a,b),c)(a, b, c) = ((a, b), c).

Explanations

L1 human

A point in the plane is an ordered pair of coordinates. The points (1,2)(1, 2) and (2,1)(2, 1) are different, so the order matters. The Kuratowski definition only shows that sets can model pairs.

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Sources

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