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 '15 at 18:31

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 '13 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 '13 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 '13 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 '14 at 23:14

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 Sep 5 '14 at 21:19
  • $\begingroup$ Or $$\bigcirc\hspace{-14pt}\iint$$ $\endgroup$ – Antonio Vargas Sep 6 '14 at 7:22
  • $\begingroup$ @AsafKaragila, this is the first time I saw the notation $\mathrm{d}\ xy$. I usually see $\mathrm{d}x\,\mathrm{d}y$. $\endgroup$ – Joel Reyes Noche Sep 6 '14 at 7:59
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    $\begingroup$ For me, these circles are off-center. $\endgroup$ – GEdgar Sep 6 '14 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 Sep 6 '14 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 '14 at 17:02
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    $\begingroup$ Off-center picture: i.stack.imgur.com/WUtQc.jpg $\endgroup$ – GEdgar Sep 6 '14 at 17:25
  • $\begingroup$ @GEdgar, are you using the STIX font, by chance? $\endgroup$ – Antonio Vargas Sep 7 '14 at 15:12
  • $\begingroup$ @AntonioVargas: Yes, it says: Using local STIX fonts ... Safari 7.0.6 $\endgroup$ – GEdgar Sep 7 '14 at 18:33
  • $\begingroup$ 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 '15 at 5:28
  • $\begingroup$ Another option: $$\subset\!\!\supset\kern-1.65em\iint$$ Idea taken from this post. $\endgroup$ – Ruslan Dec 28 '19 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 '19 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 '19 at 9:53

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