The term "integral" can refer to a number of different concepts in mathematics. The most common meaning is the the fundamenetal object of calculus corresponding to summing infinitesimal pieces to find the content of a continuous region. Other uses of "integral" include values that always take on integer values (e.g., integral embedding, integral graph), mathematical objects for which integers form basic examples (e.g., integral domain), and particular values of an equation (e.g., integral curve),
In calculus, an integral is a mathematical object that can be interpreted as an area or a generalization of area. Integrals, together with derivatives, are the fundamental objects of calculus. Other words for integral include antiderivative and primitive. The process of computing an integral is called integration (a more archaic term for integration is quadrature), and the approximate computation of an integral is termed numerical integration.
The Riemann integral is the simplest integral definition and the only one usually encountered in physics and elementary calculus. In fact, according to Jeffreys and Jeffreys (1988, p. 29), "it appears that cases where these methods [i.e., generalizations of the Riemann integral] are applicable and Riemann's [definition of the integral] is not are too rare in physics to repay the extra difficulty."
The Riemann integral of the function 
over 
from 
to 
is written
 |
(1)
|
Note that if 
,
the integral is written simply
 |
(2)
|
as opposed to 
.
Every definition of an integral is based on a particular measure. For instance, the Riemann integral is based on
Jordan measure, and the Lebesgue
integral is based on Lebesgue measure. Moreover,
depending on the context, any of a variety of other integral notations may be used.
For example, the Lebesgue integral of an integrable
function 
over a set 
which is measurable with respect to a measure 
is often written
 |
(3)
|
In the event that the set 
in () is an interval ![X=[a,b]](/images/equations/Integral/Inline11.svg)
,
the "subscript-superscript" notation from (2) is usually
adopted. Another generalization of the Riemann integral is the Stieltjes
integral, where the integrand function 
defined on a closed interval ![I=[a,b]](/images/equations/Integral/Inline13.svg)
can be integrated against a real-valued bounded function
defined on 
, the result of which has the form
 |
(4)
|
or equivalently
 |
(5)
|
Yet another scenario in which the notation may change comes about in the study of differential geometry, throughout which
the integrand 
is considered a more general differential
k-form 
and can be integrated on a set 
using either of the equivalent notations
 |
(6)
|
where 
is the above-mentioned Lebesgue measure.
Worth noting is that the notation on the left-hand side of equation () is similar
to that in expression () above.
There are two classes of (Riemann) integrals: definite integrals such as (5), which have upper and lower limits, and indefinite integrals, such as
 |
(7)
|
which are written without limits. The first fundamental theorem of calculus allows definite integrals
to be computed in terms of indefinite integrals,
since if 
is the indefinite integral for 
, then
 |
(8)
|
What's more, the first fundamental theorem of calculus can be rewritten more generally in terms of differential
forms (as in () above) to say that the integral
of a differential form 
over the boundary 
of some orientable
manifold 
is equal to the exterior derivative 
of 
over the interior of 
, i.e.,
 |
(9)
|
Written in this form, the first fundamental theorem of calculus is known as Stokes' Theorem.
Since the derivative of a constant is zero, indefinite integrals are defined only up to an arbitrary constant of integration 
, i.e.,
 |
(10)
|
Wolfram Research maintains a web site http://integrals.wolfram.com/ that can find the indefinite integral of many common (and not so common) functions.
Differentiating integrals leads to some useful and powerful identities. For instance, if 
is continuous, then
 |
(11)
|
which is the first fundamental theorem of calculus. Other derivative-integral identities include
 |
(12)
|
 |
(13)
|
(Kaplan 1992, p. 275), its generalization
 |
(14)
|
(Kaplan 1992, p. 258), and
![d/(dx)int_a^xf(x,t)dt=1/(x-a)int_a^x[(x-a)partial/(partialx)f(x,t)+(t-a)partial/(partialt)f(x,t)+f(x,t)]dt,](/images/equations/Integral/NumberedEquation15.svg) |
(15)
|
as can be seen by applying (14) on the left side of (15) and using partial integration.
Other integral identities include
 |
(16)
|
 |
(17)
|
 |  |  |
(18)
|
 |  |  |
(19)
|
and the amusing integral identity
 |
(20)
|
where 
is any function and
 |
(21)
|
as long as 
and 
is real (Glasser 1983).
Integrals with rational exponents can often be solved by making the substitution 
, where 
is the least common multiple
of the denominator of the exponents.
