# What is this symbol $\int$ called?

Title says it all (I think). I'm betting it's something to do with Standard Integrals though. What is this symbol "$\int$" called?

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I thinks it's "S"! but is anonymous. – user59671 Feb 10 '13 at 2:33
For future reference, you can learn a lot about a strange character by pasting it into Wikipedia. For example: , א, . – Rahul Feb 10 '13 at 3:38
I've heard it referred to as "long ess" before. A perhaps related query is "what is the symbol $\partial$ called?", and I have heard this one called "round dee" several times. – MPW Jul 28 '14 at 23:34

It is simply known as the "integral sign" or "integral symbol"; here is the Wikipedia article.

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See Integral Symbol. That is, it's usually called the "integral symbol".

For its origins:

"∫ symbol $\int$ is used to denote the integral in mathematics. The notation was introduced by the German mathematician Gottfried Wilhelm Leibniz towards the end of the 17th century. The symbol was based on the ſ (long s) character, and was chosen because Leibniz thought of the integral as an infinite sum of infinitesimal summands. See long s for more details on the history of ſ."

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I once had the library dig up the microfilm of the paper in which Leibniz supposedly first used the $\int$ symbol, I think in Acta Eruditorum, and I was gravely disappointed to see that it was not actually an $\int$, but only a regular ſ. I would be interested to know who first used the large, typographically distinct $\int$ sign. – MJD Feb 10 '13 at 4:36

As others have already mentioned it is usually called the "integral sign" or the "integral symbol" or just "integral" for short.

Maybe more important is how the symbol shows up in context and how then to read it out loud.

For example $$\int f(x)\;dx$$ can be read as "the integral of $f$". Or: "the indefinite integral of $f$". With both of these it is fairly common to also say $f$ of $x$ instead of just $f$.

Note that even though the symbol might be called "integral" we usually will not use it in a sentence like "and then we take the $\int$ of the function and get.

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This is the Leibniz notation for integral coming from the latin "summa" for sum (it look like a slanted S).

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-1 for just saying what that symbol is without addressing what it's called. $\:$ – Ricky Demer Feb 10 '13 at 2:26
@RickyDemer What is the sum in my answer then? – Learner Feb 10 '13 at 2:27
The sum in your answer is where it comes from. $\:$ – Ricky Demer Feb 10 '13 at 2:30
@RickyDemer There is also "for integral" and the link. How terrible of me to write in such a style !!!! – Learner Feb 10 '13 at 2:36
@Ricky: "This is the Leibniz notation for integral" seems to say what the name of the symbol is and the linked page explains it anyway, so I don't understand your explanation of the downvote. – Martin Feb 10 '13 at 2:37

This (physics) book (p. 49) says: "The integral sign, called summa, $\int$, [...]" (their emphasis), which, surprisingly, hasn't yet been suggested (although one answer got tantalisingly close).

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