In
mathematics, the Cauchy integral theorem (also known as the Cauchy–Goursat theorem) in
complex analysis, named after
Augustin-Louis Cauchy (and
Édouard Goursat
Édouard Jean-Baptiste Goursat (21 May 1858 – 25 November 1936) was a French mathematician, now remembered principally as an expositor for his ''Cours d'analyse mathématique'', which appeared in the first decade of the twentieth century. It se ...
), is an important statement about
line integral
In mathematics, a line integral is an integral where the function to be integrated is evaluated along a curve. The terms ''path integral'', ''curve integral'', and ''curvilinear integral'' are also used; ''contour integral'' is used as well, al ...
s for
holomorphic functions in the
complex plane. Essentially, it says that if
is holomorphic in a
simply connected domain
Domain may refer to:
Mathematics
*Domain of a function, the set of input values for which the (total) function is defined
**Domain of definition of a partial function
**Natural domain of a partial function
**Domain of holomorphy of a function
* Do ...
Ω, then for any simply closed contour
in Ω, that contour integral is zero.
Statement
Fundamental theorem for complex line integrals
If is a holomorphic function on an open
region
In geography, regions, otherwise referred to as zones, lands or territories, are areas that are broadly divided by physical characteristics ( physical geography), human impact characteristics ( human geography), and the interaction of humanity an ...
, and
is a curve in from
to
then,
Also, when has a single-valued antiderivative in an open region , then the path integral
is path independent for all paths in .
Formulation on simply connected regions
Let
be a
simply connected open
Open or OPEN may refer to:
Music
* Open (band), Australian pop/rock band
* The Open (band), English indie rock band
* ''Open'' (Blues Image album), 1969
* ''Open'' (Gotthard album), 1999
* ''Open'' (Cowboy Junkies album), 2001
* ''Open'' ( ...
set, and let
be a
holomorphic function. Let
be a smooth closed curve. Then:
(The condition that
be
simply connected means that
has no "holes", or in other words, that the
fundamental group of
is trivial.)
General formulation
Let
be an
open set
In mathematics, open sets are a generalization of open intervals in the real line.
In a metric space (a set along with a distance defined between any two points), open sets are the sets that, with every point , contain all points that are su ...
, and let
be a
holomorphic function. Let
be a smooth closed curve. If
is
homotopic
In topology, a branch of mathematics, two continuous functions from one topological space to another are called homotopic (from grc, ὁμός "same, similar" and "place") if one can be "continuously deformed" into the other, such a deforma ...
to a constant curve, then:
(Recall that a curve is
homotopic
In topology, a branch of mathematics, two continuous functions from one topological space to another are called homotopic (from grc, ὁμός "same, similar" and "place") if one can be "continuously deformed" into the other, such a deforma ...
to a constant curve if there exists a smooth
homotopy
In topology, a branch of mathematics, two continuous functions from one topological space to another are called homotopic (from grc, ὁμός "same, similar" and "place") if one can be "continuously deformed" into the other, such a defor ...
(within
) from the curve to the constant curve. Intuitively, this means that one can shrink the curve into a point without exiting the space.) The first version is a special case of this because on a
simply connected set, every closed curve is
homotopic
In topology, a branch of mathematics, two continuous functions from one topological space to another are called homotopic (from grc, ὁμός "same, similar" and "place") if one can be "continuously deformed" into the other, such a deforma ...
to a constant curve.
Main example
In both cases, it is important to remember that the curve
does not surround any "holes" in the domain, or else the theorem does not apply. A famous example is the following curve:
which traces out the unit circle. Here the following integral:
is nonzero. The Cauchy integral theorem does not apply here since
is not defined at
. Intuitively,
surrounds a "hole" in the domain of
, so
cannot be shrunk to a point without exiting the space. Thus, the theorem does not apply.
Discussion
As
Édouard Goursat
Édouard Jean-Baptiste Goursat (21 May 1858 – 25 November 1936) was a French mathematician, now remembered principally as an expositor for his ''Cours d'analyse mathématique'', which appeared in the first decade of the twentieth century. It se ...
showed, Cauchy's integral theorem can be proven assuming only that the complex derivative
exists everywhere in
. This is significant because one can then prove
Cauchy's integral formula
In mathematics, Cauchy's integral formula, named after Augustin-Louis Cauchy, is a central statement in complex analysis. It expresses the fact that a holomorphic function defined on a disk is completely determined by its values on the boundary ...
for these functions, and from that deduce these functions are
infinitely differentiable
In mathematical analysis, the smoothness of a function is a property measured by the number of continuous derivatives it has over some domain, called ''differentiability class''. At the very minimum, a function could be considered smooth if ...
.
The condition that
be
simply connected means that
has no "holes" or, in
homotopy
In topology, a branch of mathematics, two continuous functions from one topological space to another are called homotopic (from grc, ὁμός "same, similar" and "place") if one can be "continuously deformed" into the other, such a defor ...
terms, that the
fundamental group of
is trivial; for instance, every open disk
, for
, qualifies. The condition is crucial; consider