# Command for Big Omega [duplicate]

Possible Duplicate:
How to look up a math symbol?

How can I the code for "big omega", NOT \omega which is little omega. I want the symbol that's used in big-O notation.

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## marked as duplicate by Werner, Caramdir, Jukka Suomela, lockstep, Gonzalo MedinaOct 7 '11 at 22:23

You're probably looking for \mathcal{O} and/or \Omega. –  Werner Oct 7 '11 at 20:20
Duplicate: How to look up a math symbol? –  Werner Oct 7 '11 at 20:26
Note that \omega is also used in big-O (i.e. asymptotic) notation. –  Niel de Beaudrap Oct 7 '11 at 20:58
@JukkaSuomela: NP, but I am not 100% sure that all the OP wanted was \Omega (although that is the title). In the text he clearly indicates that he is looking for big-O notation. –  Peter Grill Oct 7 '11 at 22:48

Perhaps you are looking for something like this:

\documentclass{article}
\usepackage{amsmath}
\newcommand{\BigO}[1]{\ensuremath{\operatorname{O}\bigl(#1\bigr)}}
\begin{document}
The symbol for Big-O notation:
\BigO{n}
\end{document}


If you want the brackets to resize automatically you can use:

\newcommand{\BigO}[1]{\ensuremath{\operatorname{O}\left(#1\right)}}

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Strange. If I use your command, the O is not typeset in italics exactly as you show in the image. I have checked both Introduction to Algorithms and The C++ Programming Language and they both typeset the big O in italics. Is it correct to typeset big O in italics? –  Ali May 21 '14 at 22:44
@Ali: I don't know if the symbol should be in italics or not -- that is not a TeX type question. I suspect the reason yours is not in italics is due to some setup or package related issue. I would suggest you post a new question and include a MWE including \documentclass and the appropriate packages that reproduces the problem, and you can reference this question. –  Peter Grill May 22 '14 at 5:53
There is no problem to be reproduced. The question is: Should it be typeset in italics or not? By the way, I get exactly what you got: it is not in italics and that is exactly what I see in the image in your answer. –  Ali May 22 '14 at 11:55

It's quite easy: \Omega. Also the other big letters just begin with a capital letter, e.g. \Delta, \Pi. I can also recommend DeTeXify, there you can draw something with your mouse, and and engine tries to recognize what you drew and presents some suggestions.

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An up-front comment (not directly an answer, but I'll get to that soon): The Greek "little" o, or omicron (as opposed to the "big" o, i.e., omega), is identical in appearance -- in both its lowercase and uppercase versions -- to the Roman letters "o" and "O". Hence there are no TeX commands for $\omicron$ and $\Omicron$.

To denote the asymptotic order of magnitudes "little-oh" and "big-Oh", respectively, one frequently encounters $o$ and $O$ (i.e., using TeX's math-italics font) and -- for big-Oh only -- $\mathcal{O}$. Most math fonts (including Computer/Latin Modern) do not have lowercase mathcal letters, therefore one generally does not see $\mathcal{o}$ for "little-oh".

Incidentally, the mtpro2 package -- when loaded with the mtpcal option -- does have a symbol for $\mathcal{o}$. Unfortunately, the package is not free, but its cost (for noncommercial licenses) is very reasonable. The first line in the image below shows the results of $o$ and $O$, and the second line shows the results of $\mathcal{o}$ and $\mathcal{O}$ commands produced with the mtpro2 package loaded.

Finally, I must confess that I have not yet seen the symbol $\Omega$ (uppercase-Omega) used in asymptotic "big-Oh" notation, but that may be just be a consequence of my lack of erudition.

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$\Omega$ is used for lower bounds. E.g. all comparison sorts run in Omega(n log n) time –  kahen Oct 7 '11 at 22:08
@kahen: thanks! –  Mico Oct 7 '11 at 22:22

\newcommand{\bigO}{\ensuremath{\mathcal{O}}}% big-O notation/symbol

to your document (preamble) and then you can use \bigO. If you want to use \Omega instead to represent your biig-O notation, then add
\newcommand{\bigO}{\ensuremath{\Omega}}% big-O notation/symbol