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In other words, this:

double loop integral

According to Wikipedia, the code is \oiint, but it doesn't seem to work on this site.

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    $\begingroup$ I think $$\oint$$ is just ok for alternative. I have seen such notation on Griffith's electromagnetics book, where $\oint$ integral applies to both loops and closed surfaces, sometimes even bulks. After all, you can tell from the $d$ part, or the particular context whether the integral is a line, surface or a bulk one. $\endgroup$
    – Vim
    May 11, 2015 at 18:31

3 Answers 3

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This is off topic because it pertains to MathJax, which is a third party extension for rendering TeX on websites. Nonetheless, I'll answer it for you.

MathJax does not support this command. You will have to use the unicode directly, using the \unicode command. A table of relevant unicode commands is here; the one you seek can be obtained by \unicode{x222F}: $\large\unicode{x222F}$.

A list of commands MathJax supports is available on their site, namely here.

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    $\begingroup$ It's important to add: the MathJax webfonts do not contain the unicode character. Therefore, this relies on the hope that the reader happens to have a font installed that contains this character (such as the STIX fonts). Typesetting quality will be a bit lower with such fallback character (though not with the STIX fonts) $\endgroup$
    – Peter Krautzberger
    Jun 16, 2013 at 12:05
  • $\begingroup$ @PeterKrautzberger Good point, thanks. I've been using the STIX fonts for so long that I start forgetting about that non-obvious influence on my MathJax experience. $\endgroup$
    – Lord_Farin
    Jun 16, 2013 at 13:05
  • $\begingroup$ I can relate :) on the bright side, we're adding a number of new fonts in the next MathJax release, including STIX webfonts, so this will be less of a problem (assuming stackexchange chooses to switch to STIX webfonts which are larger.) $\endgroup$
    – Peter Krautzberger
    Jun 16, 2013 at 20:00
  • $\begingroup$ @PeterKrautzberger I wonder what you think of the alternatives in my answer and the comments below it. $\endgroup$
    – E.P.
    Sep 7, 2014 at 23:14
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Using @Ruslan's suggestion based on this post, I came up with these for both double and triple closed integrals:

\newcommand{\oiint}{{\subset\!\supset} \llap{\iint}}
\newcommand{\oiiint}{{\large{\subset\!\supset}} \llap{\iiint}}

which display as $$ {\subset\!\supset} \llap{\iint}_{\partial \Omega}{dS} $$ and $$ {\large{\subset\!\supset}} \llap{\iiint}_{\partial H}{dV} $$ respectively.

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  • $\begingroup$ {\iint\hspace{-27px}{\subset\!\supset}} $\endgroup$
    – mayi
    Aug 23 at 6:04
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Edit: Take this answer with a grain of salt. The positioning of the circles is patchy and appears to be platform-dependent. Thus, even if it looks OK in your system, it may look off-centre in other setups.

You can always try to make one yourself. For example, 

\bigcirc \!\!\!\!\!\!\!\!\!\iint_S

will produce $$\bigcirc \!\!\!\!\!\!\!\!\!\iint_S$$

This works in displayed equations and is better for that than using unicode as in Lord_Farin's answer because it scales better: the latter produces $$\unicode{x222F}$$ On the other hand, I have not found an appropriate circle command to use inline for the direct approach.

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    $\begingroup$ $$\bigcirc\kern-1.4em\iint_Sf(x,y)\mathrm d\ xy$$ looks much better. $\endgroup$
    – Asaf Karagila Mod
    Sep 5, 2014 at 21:19
  • $\begingroup$ Or $$\bigcirc\hspace{-14pt}\iint$$ $\endgroup$
    – Antonio Vargas
    Sep 6, 2014 at 7:22
  • $\begingroup$ @AsafKaragila, this is the first time I saw the notation $\mathrm{d}\ xy$. I usually see $\mathrm{d}x\,\mathrm{d}y$. $\endgroup$
    – JRN
    Sep 6, 2014 at 7:59
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    $\begingroup$ For me, these circles are off-center. $\endgroup$
    – GEdgar
    Sep 6, 2014 at 14:17
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    $\begingroup$ @Joel: This is the first time I wrote an integral for quite a long while now. Even longer for double integrals. $\endgroup$
    – Asaf Karagila Mod
    Sep 6, 2014 at 14:21
  • $\begingroup$ @GEdgar, could you take a screenshot to show how far off-center they are for you? They look like this to me: i.stack.imgur.com/DDHEE.png $\endgroup$
    – Antonio Vargas
    Sep 6, 2014 at 17:02
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    $\begingroup$ Off-center picture: i.stack.imgur.com/WUtQc.jpg $\endgroup$
    – GEdgar
    Sep 6, 2014 at 17:25
  • $\begingroup$ @GEdgar, are you using the STIX font, by chance? $\endgroup$
    – Antonio Vargas
    Sep 7, 2014 at 15:12
  • $\begingroup$ @AntonioVargas: Yes, it says: Using local STIX fonts ... Safari 7.0.6 $\endgroup$
    – GEdgar
    Sep 7, 2014 at 18:33
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    $\begingroup$ I used {\large\bigcirc}\kern-1.55em\iint to make the circle larger and adjusted the kern a little bit: $$ {\large\bigcirc}\kern-1.55em\iint $$ $\endgroup$
    – Utensil
    Oct 3, 2015 at 5:28
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    $\begingroup$ Another option: $$\subset\!\!\supset\kern-1.65em\iint$$ Idea taken from this post. $\endgroup$
    – Ruslan
    Dec 28, 2019 at 9:03
  • $\begingroup$ @Ruslan That solution looks suboptimal to me (but in any case, presenting it without the underlying code is pretty unfriendly to novices $\endgroup$
    – E.P.
    Dec 29, 2019 at 9:48
  • $\begingroup$ @E.P. the code is two mouse clicks away! But yes, the placement seems system-dependent indeed. MathRenderer→SVG seems to be more consistent though (with its own caveats, of course, like ignoration of custom colors). $\endgroup$
    – Ruslan
    Dec 29, 2019 at 9:53

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