# Notation for the union of all the sets in a family.

Does anyone know how to write the following: If $\mathcal{F}$ is a family of sets, I want the union of all $X$ in this $\mathcal{F}$.

• Just check this thread? math.stackexchange.com/questions/276104/online-mathjax-editor – Rivasa Dec 12 '13 at 20:42
• This is not a math question. Flagged. – Ahaan S. Rungta Dec 12 '13 at 20:43
• In some sense this is not a question about $\TeX$ or formatting, it's a question about the correct symbol and syntax to indicate the union of a family of sets. I don't see it so off-topic. Maybe it could be reworded... – rewritten Dec 12 '13 at 22:06
• If the question is reworded so that it asks for the current notation for union of family of sets, then I say reopen it. But if it specifically is about one can typeset something in LaTeX, then I say migrate it. Maybe the OP could scan a picture of what he is trying to do in LaTeX, then the Tex-people would know what the best way to do that is. – Thomas Dec 12 '13 at 22:31
• This looks like more of a math notation question than a $\TeX$ question. The Help Center is unclear about whether such a question belongs here. – Stefan Smith Dec 13 '13 at 1:01

This probably should be migrated to tex.SE.

But:

$$\bigcup_{X\in \mathcal{F}} X$$ would work.

• But would the typographers there know about the math background of th equestion? – Hagen von Eitzen Dec 12 '13 at 20:44
• @HagenvonEitzen: Maybe you are right. I would guess that they would. – Thomas Dec 12 '13 at 20:44

Use $\bigcup_{X \in \mathcal F} X$ or simply $\bigcup \mathcal F$.

• In code this would be $\bigcup_{x\in\mathcal{F}}X$ or $\bigcup\mathcal{F}$. – egreg Dec 12 '13 at 20:53

Use $\bigcup_i X_i$, if $\mathcal F=\{X_1,X_2,\dots\}$.

• How do you know that $\mathcal{F}$ is countable? – Thomas Dec 12 '13 at 20:43
• I did not know it: I thought that the OP needed the union symbol, after all...instead of counting the "i"'s one can write $X\in\mathcal F$ under the union. – Avitus Dec 12 '13 at 20:45