# Integral sign with a circle on it

**URL:** <https://boards.straightdope.com/t/integral-sign-with-a-circle-on-it/523768>\
**Category:** Factual Questions\
**Created:** [January 5, 2010, 6:10pm UTC](https://boards.straightdope.com/t/integral-sign-with-a-circle-on-it/523768 "2010-01-05T18:10:00Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\
**Post date:** [January 5, 2010, 6:10pm UTC](https://boards.straightdope.com/t/integral-sign-with-a-circle-on-it/523768/1 "2010-01-05T18:10:00Z")

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What does it mean? It looks like a normal integral sign, with an open circle super-imposed in the middle of the squiggle.

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**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\
**Post date:** [January 5, 2010, 6:16pm UTC](https://boards.straightdope.com/t/integral-sign-with-a-circle-on-it/523768/2 "2010-01-05T18:16:57Z")

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[Integral over a closed path.](http://en.wikipedia.org/wiki/Line_integral#Complex_line_integral)

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**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\
**Post date:** [January 5, 2010, 8:09pm UTC](https://boards.straightdope.com/t/integral-sign-with-a-circle-on-it/523768/3 "2010-01-05T20:09:45Z")

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Also known as a [Contour Integral](http://en.wikipedia.org/wiki/Methods_of_contour_integration).

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**Author:** ![OldGuy](https://avatars.discourse-cdn.com/v4/letter/o/3bc359/32.png) [@OldGuy](https://boards.straightdope.com/u/OldGuy)\
**Post date:** [January 5, 2010, 8:15pm UTC](https://boards.straightdope.com/t/integral-sign-with-a-circle-on-it/523768/4 "2010-01-05T20:15:36Z")

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> [@tomndebb](#):
>
> Also known as a [Contour Integral](http://en.wikipedia.org/wiki/Methods_of_contour_integration).

A contour integral can be over any contour not necessarily a closed path (which starts and ends at the same spot. So all integrals over closed paths are contour integrals (as are all normal real line integrals whose path is just some portion of the real line), but not all contour integrals are integrals over closed paths. The symbol the OP asked about is usually just for closed paths.

BTW a double integral with a superimposed circle (or often oval) is an integral over a closed surface. e.g., an integral of a function over an entire sphere.

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**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\
**Post date:** [January 5, 2010, 8:34pm UTC](https://boards.straightdope.com/t/integral-sign-with-a-circle-on-it/523768/5 "2010-01-05T20:34:59Z")

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Ugghhh.

Those things can “take you to the limit” coming and going 🙂

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**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\
**Post date:** [January 5, 2010, 8:42pm UTC](https://boards.straightdope.com/t/integral-sign-with-a-circle-on-it/523768/6 "2010-01-05T20:42:43Z")

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What does it mean?

It mean’s you’re taking second semester engineering physics, Maxwell’s equations, and your Calculus class isn’t keeping up. :eek:

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**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\
**Post date:** [January 5, 2010, 8:45pm UTC](https://boards.straightdope.com/t/integral-sign-with-a-circle-on-it/523768/7 "2010-01-05T20:45:29Z")

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But relax, you won’t need it ever again after you take the final.

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**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\
**Post date:** [January 5, 2010, 11:53pm UTC](https://boards.straightdope.com/t/integral-sign-with-a-circle-on-it/523768/8 "2010-01-05T23:53:41Z")

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> [@ElvisL1ves](#):
>
> But relax, you won’t need it ever again after you take the final.

Yep. I took all sorts of high level math classes in college, worked in a highly technical field in Silicon Valley, and I think the most difficult calculation I ever had to do on the job was to take an average. 🙂