11
$\begingroup$

How would one denote the set of all subsets of $A$ which have size $2$?

Would $\binom{A}{2}$

be a good idea?

  • 0
    I made a mistake in my notation I meant $\{X\in A^{|2|}:|X|=2\}$2012-02-24

2 Answers 2

15

$\dbinom A 2$ is standard notation for the set of all size-$2$ subsets of a set $A$, in the usage of combinatorialists.

  • 3
    The _number_ of unordered pairs from $A$ is $\dbinom{|A|}{2}$ (where $|A|$ is the number of members of the set $A$). The _set_ of unordered pairs from $A$ is $\dbinom{A}{2}$.2012-02-25
7

In set theory it can be often denoted as $[A]^2=\{\{a,b\}:a,b\in A, a\neq b\}$.

  • 0
    Add condition that $a\neq b$.2012-02-24