int (LaTeX symbol)
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TeX has \int
as the integral sign. Integral expressions are formed from the use of sub and superscript, the judicious use of spacing, and simply writing out the differential. For example, a standard integral in LaTeX looks like

 \int_a^b \! f(x) \, \mathrm{d}x.
Note the use of \mathrm
to make a Roman "d" which distinguishes it from the product of variables d and x. Note, too, the use of \! to bring the function closer to the integral sign and the \, to push the differential farther away. Without them, the integral looks like
 \int_a^b f(x) dx,
which, although logically identical, is less legible and rankles the aesthetic sensibilities of many.
Examples Edit
You can also treat the integral as a sumclass symbol with the \limit
command. This is most useful for double and triple integrals. For example,

\iint\limits_D \, \dif x\,\dif y \quad \iiint\limits_E \, \dif x\,\dif y\,\mathrm{d}z
where D and E are regions that satisfy the requirements.
And let's not forget the closed path integral.

\oint \! \nabla f \, \dif t = 0