Editing Union (set theory) (section)

== Notation ==
The notation for the general concept can vary considerably.  For a finite union of sets <math>S_1, S_2, S_3, \dots , S_n</math> one often writes <math>S_1 \cup S_2 \cup S_3 \cup \dots \cup S_n</math> or <math display="inline">\bigcup_{i=1}^n S_i</math>.  Various common notations for arbitrary unions include <math display="inline">\bigcup \mathbf{M}</math>, <math display="inline">\bigcup_{A\in\mathbf{M}} A</math>, and <math display="inline">\bigcup_{i\in I} A_{i}</math>. The last of these notations refers to the union of the collection <math>\left\{A_i : i \in I\right\}</math>, where ''I'' is an [[index set]] and <math>A_i</math> is a set for every {{tmath|1= i \in I }}. In the case that the index set ''I'' is the set of [[natural number]]s, one uses the notation <math display="inline">\bigcup_{i=1}^{\infty} A_{i}</math>, which is analogous to that of the [[infinite sum]]s in series.<ref name=":1" />

When the symbol "∪" is placed before other symbols (instead of between them), it is usually rendered as a larger size.

=== Notation encoding ===
In [[Unicode]], union is represented by the character {{unichar|222A|Union}}.<ref>{{cite web |title=The Unicode Standard, Version 15.0 – Mathematical Operators – Range: 2200–22FF |url=https://www.unicode.org/charts/PDF/U2200.pdf |website=[[Unicode]] |page=3}}</ref> In [[TeX]], <math>\cup</math> is rendered from <code>\cup</code> and <math display="inline">\bigcup</math> is rendered from <code>\bigcup</code>.