Contour integration explained

In the mathematical field of complex analysis, contour integration is a method of evaluating certain integrals along paths in the complex plane.^{[1] [2] [3]}

Contour integration is closely related to the calculus of residues,^[4] a method of complex analysis.

One use for contour integrals is the evaluation of integrals along the real line that are not readily found by using only real variable methods.^[5]

Contour integration methods include:

• direct integration of a complex-valued function along a curve in the complex plane;
• application of the Cauchy integral formula; and
• application of the residue theorem.

One method can be used, or a combination of these methods, or various limiting processes, for the purpose of finding these integrals or sums.

Curves in the complex plane

In complex analysis a contour is a type of curve in the complex plane. In contour integration, contours provide a precise definition of the curves on which an integral may be suitably defined. A curve in the complex plane is defined as a continuous function from a closed interval of the real line to the complex plane:

z:[a,b]\to\C .

This definition of a curve coincides with the intuitive notion of a curve, but includes a parametrization by a continuous function from a closed interval. This more precise definition allows us to consider what properties a curve must have for it to be useful for integration. In the following subsections we narrow down the set of curves that we can integrate to include only those that can be built up out of a finite number of continuous curves that can be given a direction. Moreover, we will restrict the "pieces" from crossing over themselves, and we require that each piece have a finite (non-vanishing) continuous derivative. These requirements correspond to requiring that we consider only curves that can be traced, such as by a pen, in a sequence of even, steady strokes, which stop only to start a new piece of the curve, all without picking up the pen.^[6]

Directed smooth curves

Contours are often defined in terms of directed smooth curves.^[6] These provide a precise definition of a "piece" of a smooth curve, of which a contour is made.

A smooth curve is a curve

z:[a,b]\to\C with a non-vanishing, continuous derivative such that each point is traversed only once (is one-to-one), with the possible exception of a curve such that the endpoints match (z(a) = z(b) ). In the case where the endpoints match the curve is called closed, and the function is required to be one-to-one everywhere else and the derivative must be continuous at the identified point (z'(a) = z'(b) ). A smooth curve that is not closed is often referred to as a smooth arc.^[6]

The parametrization of a curve provides a natural ordering of points on the curve:

z(x) comes before z(y) if x<y . This leads to the notion of a directed smooth curve. It is most useful to consider curves independent of the specific parametrization. This can be done by considering equivalence classes of smooth curves with the same direction. A directed smooth curve can then be defined as an ordered set of points in the complex plane that is the image of some smooth curve in their natural order (according to the parametrization). Note that not all orderings of the points are the natural ordering of a smooth curve. In fact, a given smooth curve has only two such orderings. Also, a single closed curve can have any point as its endpoint, while a smooth arc has only two choices for its endpoints.

Contours

Contours are the class of curves on which we define contour integration. A contour is a directed curve which is made up of a finite sequence of directed smooth curves whose endpoints are matched to give a single direction. This requires that the sequence of curves

\gamma _{1,...,\gamma} _{n} be such that the terminal point of \gamma _{i} coincides with the initial point of \gamma _{i+1} for all i such that 1 \leq i<n . This includes all directed smooth curves. Also, a single point in the complex plane is considered a contour. The symbol + is often used to denote the piecing of curves together to form a new curve. Thus we could write a contour \Gamma that is made up of n curves as$$\backslash Gamma\; =\; \backslash gamma\_1\; +\; \backslash gamma\_2\; +\; \backslash cdots\; +\; \backslash gamma\_n.$$

Contour integrals

f:\C\to\C is a generalization of the integral for real-valued functions. For continuous functions in the complex plane, the contour integral can be defined in analogy to the line integral by first defining the integral along a directed smooth curve in terms of an integral over a real valued parameter. A more general definition can be given in terms of partitions of the contour in analogy with the partition of an interval and the Riemann integral. In both cases the integral over a contour is defined as the sum of the integrals over the directed smooth curves that make up the contour.

For continuous functions

To define the contour integral in this way one must first consider the integral, over a real variable, of a complex-valued function. Let

f:\R\to\C be a complex-valued function of a real variable, t . The real and imaginary parts of f are often denoted as u(t) and v(t) , respectively, so that$$f(t)\; =\; u(t)\; +\; iv(t).$$Then the integral of the complex-valued function f over the interval [a,b] is given by

Now, to define the contour integral, let

f:\C\to\C be a continuous function on the directed smooth curve \gamma . Let z:[a,b]\to\C be any parametrization of \gamma that is consistent with its order (direction). Then the integral along \gamma is denoted$$\backslash int\_\backslash gamma\; f(z)\backslash ,\; dz\backslash ,$$and is given by^[6] $$\backslash int\_\backslash gamma\; f(z)\; \backslash ,\; dz\; :=\; \backslash int\_a^b\; f\backslash big(z(t)\backslash big)\; z\text{'}(t)\; \backslash ,\; dt.$$

This definition is well defined. That is, the result is independent of the parametrization chosen. In the case where the real integral on the right side does not exist the integral along

\gamma is said not to exist.

As a generalization of the Riemann integral

The generalization of the Riemann integral to functions of a complex variable is done in complete analogy to its definition for functions from the real numbers. The partition of a directed smooth curve

\gamma is defined as a finite, ordered set of points on \gamma . The integral over the curve is the limit of finite sums of function values, taken at the points on the partition, in the limit that the maximum distance between any two successive points on the partition (in the two-dimensional complex plane), also known as the mesh, goes to zero.

Direct methods

Direct methods involve the calculation of the integral through methods similar to those in calculating line integrals in multivariate calculus. This means that we use the following method:

• parametrizing the contour

The contour is parametrized by a differentiable complex-valued function of real variables, or the contour is broken up into pieces and parametrized separately.

• substitution of the parametrization into the integrand

Substituting the parametrization into the integrand transforms the integral into an integral of one real variable.

• direct evaluation

The integral is evaluated in a method akin to a real-variable integral.

Example

A fundamental result in complex analysis is that the contour integral of is, where the path of the contour is taken to be the unit circle traversed counterclockwise (or any positively oriented Jordan curve about 0). In the case of the unit circle there is a direct method to evaluate the integral$$\backslash oint\_C\; \backslash frac\backslash ,dz.$$

In evaluating this integral, use the unit circle as a contour, parametrized by, with, then and$$\backslash oint\_C\; \backslash frac\backslash ,dz\; =\; \backslash int\_0^\; \backslash frac\; ie^\backslash ,dt\; =\; i\backslash int\_0^\; 1\; \backslash ,\; dt\; =\; i\; \backslash ,\; t\backslash Big|\_0^\; =\; \backslash left(2\backslash pi-0\backslash right)i\; =\; 2\backslash pi\; i$$

which is the value of the integral. This result only applies to the case in which z is raised to power of -1. If the power is not equal to -1, then the result will always be zero.

Applications of integral theorems

Applications of integral theorems are also often used to evaluate the contour integral along a contour, which means that the real-valued integral is calculated simultaneously along with calculating the contour integral.

Integral theorems such as the Cauchy integral formula or residue theorem are generally used in the following method:

• a specific contour is chosen:

The contour is chosen so that the contour follows the part of the complex plane that describes the real-valued integral, and also encloses singularities of the integrand so application of the Cauchy integral formula or residue theorem is possible

• application of Cauchy's integral theorem

The integral is reduced to only an integration around a small circle about each pole.

• application of the Cauchy integral formula or residue theorem

Application of these integral formulae gives us a value for the integral around the whole of the contour.

• division of the contour into a contour along the real part and imaginary part

The whole of the contour can be divided into the contour that follows the part of the complex plane that describes the real-valued integral as chosen before (call it), and the integral that crosses the complex plane (call it). The integral over the whole of the contour is the sum of the integral over each of these contours.

• demonstration that the integral that crosses the complex plane plays no part in the sum

If the integral can be shown to be zero, or if the real-valued integral that is sought is improper, then if we demonstrate that the integral as described above tends to 0, the integral along will tend to the integral around the contour .

• conclusion

If we can show the above step, then we can directly calculate, the real-valued integral.

Example 1

Consider the integral$$\backslash int\_^\backslash infty\; \backslash frac\backslash ,dx,$$

To evaluate this integral, we look at the complex-valued function$$f(z)=\backslash frac$$

which has singularities at and . We choose a contour that will enclose the real-valued integral, here a semicircle with boundary diameter on the real line (going from, say, to) will be convenient. Call this contour .

There are two ways of proceeding, using the Cauchy integral formula or by the method of residues:

Using the Cauchy integral formula

Note that:$$\backslash oint\_C\; f(z)\backslash ,dz\; =\; \backslash int\_^a\; f(z)\backslash ,dz\; +\; \backslash int\_\backslash text\; f(z)\backslash ,dz$$thus$$\backslash int\_^a\; f(z)\backslash ,dz\; =\; \backslash oint\_C\; f(z)\backslash ,dz\; -\; \backslash int\_\backslash text\; f(z)\backslash ,dz$$

Furthermore, observe that$$f(z)=\backslash frac=\backslash frac.$$

Since the only singularity in the contour is the one at , then we can write$$f(z)=\backslash frac,$$

which puts the function in the form for direct application of the formula. Then, by using Cauchy's integral formula,$$\backslash oint\_C\; f(z)\backslash ,dz\; =\; \backslash oint\_C\; \backslash frac\backslash ,dz\; =\; 2\backslash pi\; i\; \backslash ,\; \backslash left.\backslash frac\; \backslash frac\backslash right|\_\; =2\; \backslash pi\; i\; \backslash left[\backslash frac\{-2\}\{(z+i)^3\}\backslash right]\_\; =\backslash frac$$

We take the first derivative, in the above steps, because the pole is a second-order pole. That is, is taken to the second power, so we employ the first derivative of . If it were taken to the third power, we would use the second derivative and divide by 2!, etc. The case of to the first power corresponds to a zero order derivative—just itself.

We need to show that the integral over the arc of the semicircle tends to zero as, using the estimation lemma$$\backslash left|\backslash int\_\backslash text\; f(z)\backslash ,dz\backslash right|\; \backslash le\; ML$$

where is an upper bound on along the arc and the length of the arc. Now,$$\backslash left|\backslash int\_\backslash text\; f(z)\backslash ,dz\backslash right|\backslash le\; \backslash frac\; \backslash to\; 0\; \backslash text\; a\; \backslash to\; \backslash infty.$$So$$\backslash int\_^\backslash infty\; \backslash frac\backslash ,dx\; =\; \backslash int\_^\backslash infty\; f(z)\backslash ,dz\; =\; \backslash lim\_\; \backslash int\_^a\; f(z)\backslash ,dz\; =\; \backslash frac2.\backslash quad\backslash square$$

Using the method of residues

Consider the Laurent series of about, the only singularity we need to consider. We then have$$f(z)\; =\; \backslash frac\; +\; \backslash frac\; +\; \backslash frac\; +\; \backslash frac(z-i)\; +\; \backslash frac(z-i)^2\; +\; \backslash cdots$$

(See the sample Laurent calculation from Laurent series for the derivation of this series.)

It is clear by inspection that the residue is, so, by the residue theorem, we have$$\backslash oint\_C\; f(z)\backslash ,dz\; =\; \backslash oint\_C\; \backslash frac\backslash ,dz\; =\; 2\; \backslash pi\; i\; \backslash ,\backslash operatorname\_\; f(z)\; =\; 2\; \backslash pi\; i\; \backslash left(-\backslash frac\backslash right)=\backslash frac2\; \backslash quad\backslash square$$

Thus we get the same result as before.

Contour note

As an aside, a question can arise whether we do not take the semicircle to include the other singularity, enclosing . To have the integral along the real axis moving in the correct direction, the contour must travel clockwise, i.e., in a negative direction, reversing the sign of the integral overall.

This does not affect the use of the method of residues by series.

Example 2 – Cauchy distribution

The integral$$\backslash int\_^\backslash infty\; \backslash frac\backslash ,dx$$

(which arises in probability theory as a scalar multiple of the characteristic function of the Cauchy distribution) resists the techniques of elementary calculus. We will evaluate it by expressing it as a limit of contour integrals along the contour that goes along the real line from to and then counterclockwise along a semicircle centered at 0 from to . Take to be greater than 1, so that the imaginary unit is enclosed within the curve. The contour integral is$$\backslash int\_C\; \backslash frac\backslash ,dz.$$

Since is an entire function (having no singularities at any point in the complex plane), this function has singularities only where the denominator is zero. Since, that happens only where or . Only one of those points is in the region bounded by this contour. The residue of at is$$\backslash lim\_(z-i)f(z)\; =\; \backslash lim\_(z-i)\backslash frac\; =\; \backslash lim\_(z-i)\backslash frac\; =\; \backslash lim\_\backslash frac\; =\; \backslash frac.$$

According to the residue theorem, then, we have$$\backslash int\_C\; f(z)\backslash ,dz=2\backslash pi\; i\; \backslash operatorname\_f(z)=2\backslash pi\; i\backslash frac=\backslash pi\; e^.$$

The contour may be split into a "straight" part and a curved arc, so that$$\backslash int\_\backslash text+\backslash int\_\backslash text=\backslash pi\; e^,$$and thus$$\backslash int\_^a\; =\backslash pi\; e^-\backslash int\_\backslash text.$$

According to Jordan's lemma, if then$$\backslash int\_\backslash text\backslash frac\backslash ,dz\; \backslash rightarrow\; 0\; \backslash mbox\; a\backslash rightarrow\backslash infty.$$

Therefore, if then$$\backslash int\_^\backslash infty\; \backslash frac\backslash ,dx=\backslash pi\; e^.$$

A similar argument with an arc that winds around rather than shows that if then$$\backslash int\_^\backslash infty\; \backslash frac\backslash ,dx=\backslash pi\; e^t,$$and finally we have this:$$\backslash int\_^\backslash infty\; \backslash frac\; \backslash ,dx=\backslash pi\; e^.$$

(If then the integral yields immediately to real-valued calculus methods and its value is .)

Example 3 – trigonometric integrals

Certain substitutions can be made to integrals involving trigonometric functions, so the integral is transformed into a rational function of a complex variable and then the above methods can be used in order to evaluate the integral.

As an example, consider$$\backslash int\_^\backslash pi\; \backslash frac\; \backslash ,dt.$$

We seek to make a substitution of . Now, recall$$\backslash cos\; t\; =\; \backslash frac12\; \backslash left(e^+e^\backslash right)\; =\; \backslash frac12\; \backslash left(z+\backslash frac\backslash right)$$and$$\backslash frac\; =\; iz,\backslash \; dt\; =\; \backslash frac.$$

Taking to be the unit circle, we substitute to get:

\begin{align} \oint _{C} (1)/(1 + 3 \left( (12)/(\left(z + (1)/(z)) \right)\right) ^{2}) (dz)/(iz) & = \oint _{C} (1)/(1 + (34)/(\left(z + (1)/(z)) \right) ^{2}) (1)/(iz) dz \\ & = \oint _{C} (- i)/(z + (34)/(z\left(z + (1)/(z)) \right) ^{2}) dz \\ & = - i \oint _{C} (dz)/(z + (34)/(z\left(z) ^{2+2+ (1)/(z ^{2})} \right)) \\ & = - i \oint _{C} (dz)/(z + (34)/(\left(z) ^{3+2 z + (1)/(z)} \right)) \\ & = - i \oint _{C} (dz)/((34)/(z) ^{3+ (52)/(z + (3)/(4 z))}) \\ & = - i \oint _{C} (4)/(3 z ^{3+10 z + (3)/(z)}) dz \\ & = -4 i \oint _{C} (dz)/(3 z ^{3+10 z + (3)/(z)}) \\ & = -4 i \oint _{C} (z)/(3 z ^{4+10z} ^{2+3}) dz \\ & = -4 i \oint _{C} (z)/(i\right)\left(z - \sqrt{ 3 }i\right)\left(z + 3 \left(z + \sqrt{ 3) (i)/(\right)\left(z - \sqrt 3) (i)/(\sqrt 3) \right)}dz \\ & =- (4 i)/(3) \oint _{C} (z)/(i\right)\left(z - \sqrt{ 3 }i\right)\left(z + \left(z + \sqrt{ 3) (i)/(\right)\left(z - \sqrt 3) (i)/(\sqrt 3) \right)}dz. \end{align}

The singularities to be considered are at

\tfrac{\pm i}{\sqrt{ 3 }}. Let be a small circle about \tfrac{i}{\sqrt{ 3 }}, and be a small circle about \tfrac{ - i}{\sqrt{ 3 }}. Then we arrive at the following:

Example 3a – trigonometric integrals, the general procedure

The above method may be applied to all integrals of the type$$\backslash int\_0^\; \backslash frac\backslash ,\; dt$$

where and are polynomials, i.e. a rational function in trigonometric terms is being integrated. Note that the bounds of integration may as well be and −, as in the previous example, or any other pair of endpoints 2 apart.

The trick is to use the substitution where and hence$$\backslash frac\; \backslash ,dz\; =\; dt.$$

This substitution maps the interval to the unit circle. Furthermore,$$\backslash sin(k\; t)\; =\; \backslash frac\; =\; \backslash frac$$and$$\backslash cos(k\; t)\; =\; \backslash frac\; =\; \backslash frac$$so that a rational function in results from the substitution, and the integral becomes$$\backslash oint\_$$

=1

f(z) \frac\, dz which is in turn computed by summing the residues of inside the unit circle.

The image at right illustrates this for$$I\; =\; \backslash int\_0^\backslash frac\; \backslash frac\backslash ,\; dt,$$which we now compute. The first step is to recognize that$$I\; =\; \backslash frac14\; \backslash int\_0^\; \backslash frac\; \backslash ,dt.$$

The substitution yields$$\backslash frac\; \backslash oint\_$$

=1

\frac\, dz = \oint_

=1

\frac\, dz.

The poles of this function are at and . Of these, and are outside the unit circle (shown in red, not to scale), whereas and are inside the unit circle (shown in blue). The corresponding residues are both equal to, so that the value of the integral is$$I\; =\; 2\; \backslash pi\; i\; \backslash ;\; 2\; \backslash left(-\; \backslash frac\; i\; \backslash right)\; =\; \backslash pi\; \backslash frac.$$

Example 4 – branch cuts

Consider the real integral$$\backslash int\_0^\backslash infty\; \backslash frac\backslash ,dx.$$

We can begin by formulating the complex integral$$\backslash int\_C\; \backslash frac\backslash ,dz=I.$$

We can use the Cauchy integral formula or residue theorem again to obtain the relevant residues. However, the important thing to note is that, so has a branch cut. This affects our choice of the contour . Normally the logarithm branch cut is defined as the negative real axis, however, this makes the calculation of the integral slightly more complicated, so we define it to be the positive real axis.

Then, we use the so-called keyhole contour, which consists of a small circle about the origin of radius say, extending to a line segment parallel and close to the positive real axis but not touching it, to an almost full circle, returning to a line segment parallel, close, and below the positive real axis in the negative sense, returning to the small circle in the middle.

Note that and are inside the big circle. These are the two remaining poles, derivable by factoring the denominator of the integrand. The branch point at was avoided by detouring around the origin.

Let be the small circle of radius, the larger, with radius, then$$\backslash int\_C\; =\; \backslash int\_\backslash varepsilon^R\; +\; \backslash int\_\backslash Gamma\; +\; \backslash int\_R^\backslash varepsilon\; +\; \backslash int\_\backslash gamma.$$

It can be shown that the integrals over and both tend to zero as and, by an estimation argument above, that leaves two terms. Now since, on the contour outside the branch cut, we have gained 2 in argument along . (By Euler's identity, represents the unit vector, which therefore has as its log. This is what is meant by the argument of . The coefficient of forces us to use 2.) So

Therefore:$$\backslash int\_C\; \backslash frac\backslash ,dz=2\backslash int\_0^\backslash infty\; \backslash frac\backslash ,dx.$$

By using the residue theorem or the Cauchy integral formula (first employing the partial fractions method to derive a sum of two simple contour integrals) one obtains$$\backslash pi\; i\; \backslash left(\backslash frac-i\backslash right)=\backslash int\_0^\backslash infty\; \backslash frac\backslash ,dx\; =\; \backslash pi\backslash left(1-\backslash frac\backslash right).\backslash quad\backslash square$$

Example 5 – the square of the logarithm

This section treats a type of integral of which$$\backslash int\_0^\backslash infty\; \backslash frac\; \backslash ,\; dx$$is an example.

To calculate this integral, one uses the function$$f(z)\; =\; \backslash left\; (\backslash frac\; \backslash right)^2$$and the branch of the logarithm corresponding to .

We will calculate the integral of along the keyhole contour shown at right. As it turns out this integral is a multiple of the initial integral that we wish to calculate and by the Cauchy residue theorem we have

\begin{align} \left( \int _{R} + \int _{M} + \int _{N} + \int _{r} \right) f(z) dz = & 2 \pi i ( \operatorname{Res} _{z=i} f(z) + \operatorname{Res} _{z=-i} f(z) ) \\ = & 2 \pi i \left( - (\pi)/(4) + (1)/(16) i \pi ^{2} - (\pi)/(4) - (1)/(16) i \pi ^{2} \right) \\ = & - i \pi ^{2.} \end{align}

Let be the radius of the large circle, and the radius of the small one. We will denote the upper line by, and the lower line by . As before we take the limit when and . The contributions from the two circles vanish. For example, one has the following upper bound with the lemma:$$\backslash left|\; \backslash int\_R\; f(z)\; \backslash ,\; dz\; \backslash right|\; \backslash le\; 2\; \backslash pi\; R\; \backslash frac\; \backslash to\; 0.$$

In order to compute the contributions of and we set on and on, with :

\begin{align} - i \pi ^{2} & = \left( \int _{R} + \int _{M} + \int _{N} + \int _{r} \right) f(z) dz \\[ 6 pt] & = \left( \int _{M} + \int _{N} \right) f(z) dz && \int _{R,} \int _{r} vanish \\[ 6 pt] & =- \int ^{0} _{inf ty} \left ( (log ( - x + i\varepsilon))/(1+ ( - x + i\varepsilon) ^{2}) \right ) ^{2} dx - \int ^{inf ty} _{0} \left ( (log ( - x - i\varepsilon))/(1+ ( - x - i\varepsilon) ^{2}) \right) ^{2} dx \\[ 6 pt] & = \int ^{inf ty} _{0} \left ( (log ( - x + i\varepsilon))/(1+ ( - x + i\varepsilon) ^{2}) \right ) ^{2} dx - \int ^{inf ty} _{0} \left ( (log ( - x - i\varepsilon))/(1+ ( - x - i\varepsilon) ^{2}) \right ) ^{2} dx \\[ 6 pt] & = \int ^{inf ty} _{0} \left ( (log x + i\pi)/(1+ x ^{2}) \right ) ^{2} dx - \int ^{inf ty} _{0} \left ( (log x - i\pi)/(1+ x ^{2}) \right ) ^{2} dx && \varepsilon \to 0 \\ & = \int ^{inf ty} _{0} (( log x + i\pi) ^{2} - ( log x - i\pi) ^{2})/(\left( 1+ x ^{2\right)} ^{2}) dx \\[ 6 pt] & = \int ^{inf ty} _{0} (4 \pi i log x)/(\left( 1+ x ^{2\right)} ^{2}) dx \\[ 6 pt] & = 4 \pi i \int ^{inf ty} _{0} (log x)/(\left( 1+ x ^{2\right)} ^{2}) dx \end{align}

which gives$$\backslash int\_0^\backslash infty\; \backslash frac\; \backslash ,\; dx\; =\; -\; \backslash frac.$$

Example 6 – logarithms and the residue at infinity

We seek to evaluate$$I\; =\; \backslash int\_0^3\; \backslash frac\backslash ,dx.$$

This requires a close study of$$f(z)\; =\; z^\backslash frac34\; (3-z)^\backslash frac14.$$

We will construct so that it has a branch cut on, shown in red in the diagram. To do this, we choose two branches of the logarithm, settingand

The cut of is therefore and the cut of is . It is easy to see that the cut of the product of the two, i.e., is, because is actually continuous across . This is because when and we approach the cut from above, has the value$$r^\backslash frac34\; e^\; (3+r)^\backslash frac14\; e^\; =\; r^\backslash frac34\; (3+r)^\backslash frac14\; e^.$$

When we approach from below, has the value$$r^\backslash frac34\; e^\; (3+r)^\backslash frac14\; e^\; =\; r^\backslash frac34\; (3+r)^\backslash frac14\; e^.$$

But$$e^\; =\; e^,$$

so that we have continuity across the cut. This is illustrated in the diagram, where the two black oriented circles are labelled with the corresponding value of the argument of the logarithm used in and .

We will use the contour shown in green in the diagram. To do this we must compute the value of along the line segments just above and just below the cut.

Let (in the limit, i.e. as the two green circles shrink to radius zero), where . Along the upper segment, we find that has the value$$r^\backslash frac34\; e^\; (3-r)^\backslash frac14\; e^\; =\; i\; r^\backslash frac34\; (3-r)^\backslash frac14$$and along the lower segment,$$r^\backslash frac34\; e^\; (3-r)^\backslash frac14\; e^\; =\; r^\backslash frac34\; (3-r)^\backslash frac14.$$

It follows that the integral of along the upper segment is in the limit, and along the lower segment, .

If we can show that the integrals along the two green circles vanish in the limit, then we also have the value of, by the Cauchy residue theorem. Let the radius of the green circles be, where and, and apply the inequality. For the circle on the left, we find$$\backslash left|\; \backslash int\_\; \backslash frac\; dz\; \backslash right|\; \backslash le\; 2\; \backslash pi\; \backslash rho\; \backslash frac\; \backslash in\; \backslash mathcal\; \backslash left(\backslash rho^\backslash frac74\; \backslash right)\; \backslash to\; 0.$$

Similarly, for the circle on the right, we have$$\backslash left|\; \backslash int\_\; \backslash frac\; dz\; \backslash right|\; \backslash le\; 2\; \backslash pi\; \backslash rho\; \backslash frac\; \backslash in\; \backslash mathcal\; \backslash left(\backslash rho^\backslash frac54\; \backslash right)\; \backslash to\; 0.$$

Now using the Cauchy residue theorem, we have$$(-i\; +\; 1)\; I\; =\; -2\backslash pi\; i\; \backslash left(\backslash operatorname\_\; \backslash frac\; +\; \backslash operatorname\_\; \backslash frac\; \backslash right).$$where the minus sign is due to the clockwise direction around the residues. Using the branch of the logarithm from before, clearly$$\backslash operatorname\_\; \backslash frac\; =\; -\; 5^\backslash frac34\; e^.$$

The pole is shown in blue in the diagram. The value simplifies to$$-5^\backslash frac34\; e^\; =\; -e^\; 5^\backslash frac34\; 2^\backslash frac14.$$

We use the following formula for the residue at infinity:$$\backslash operatorname\_\; h(z)\; =\; \backslash operatorname\_\; \backslash left(-\; \backslash frac\; h\backslash left(\backslash frac\backslash right)\backslash right).$$

Substituting, we find$$\backslash frac\; =\; -z\; \backslash left(1\; +\; 5z\; +\; 5^2\; z^2\; +\; 5^3\; z^3\; +\; \backslash cdots\backslash right)$$and$$\backslash left(\backslash frac\backslash left\; (3-\backslash frac\; \backslash right)\backslash right)^\backslash frac14\; =\; \backslash frac\; (3z-1)^\backslash frac14\; =\; \backslash frace^\; (1-3z)^\backslash frac14,$$where we have used the fact that for the second branch of the logarithm. Next we apply the binomial expansion, obtaining$$\backslash frac\; e^\; \backslash left(1\; -\; 3z\; +\; 3^2\; z^2\; -\; 3^3\; z^3\; +\; \backslash cdots\; \backslash right).$$

The conclusion is that$$\backslash operatorname\_\; \backslash frac\; =\; e^\; \backslash left\; (5\; -\; \backslash frac34\; \backslash right)\; =\; e^\backslash frac.$$

Finally, it follows that the value of is$$I\; =\; 2\; \backslash pi\; i\; \backslash frac\; \backslash left(\backslash frac\; -\; 5^\backslash frac34\; 2^\backslash frac14\; \backslash right)\; =\; 2\; \backslash pi\; 2^\; \backslash left(\backslash frac\; -\; 5^\backslash frac34\; 2^\backslash frac14\; \backslash right)$$which yields$$I\; =\; \backslash frac\; \backslash left(17\; -\; 5^\backslash frac34\; 2^\backslash frac94\; \backslash right)\; =\; \backslash frac\; \backslash left(17\; -\; 40^\backslash frac34\; \backslash right).$$

Evaluation with residue theorem

Using the residue theorem, we can evaluate closed contour integrals. The following are examples on evaluating contour integrals with the residue theorem.

Using the residue theorem, let us evaluate this contour integral.$$\backslash oint\_C\; \backslash frac\backslash ,dz$$

Recall that the residue theorem states$$\backslash oint\_\; f(z)=2\backslash pi\; i\backslash cdot\; \backslash sum\backslash operatorname(f,a\_k),$$

where

\operatorname{Res} is the residue of f(z) , and the a _{k} are the singularities of f(z) lying inside the contour C (with none of them lying directly on C ).f(z) has only one pole, 0 . From that, we determine that the residue of f(z) to be \tfrac{ 1 }{ 2 }

Thus, using the residue theorem, we can determine:$$\backslash oint\_C\; \backslash frac\; dz\; =\; \backslash pi\; i.$$

Multivariable contour integrals

To solve multivariable contour integrals (i.e. surface integrals, complex volume integrals, and higher order integrals), we must use the divergence theorem. For right now, let

\nabla be interchangeable with \operatorname{Div} . These will both serve as the divergence of the vector field denoted as F . This theorem states:$$\backslash underbrace\_n\; \backslash operatorname(\backslash mathbf)\; \backslash ,\; dV\; =\; \backslash underbrace\_\; \backslash mathbf\; \backslash cdot\; \backslash mathbf\; \backslash ,\; dS$$

In addition, we also need to evaluate

\nabla ⋅ F where \nabla ⋅ F is an alternate notation of \operatorname{div} ( F ) . The divergence of any dimension can be described as

Example 1

F = \sin( 2 x) e _{x+\sin(2y)e} _{y+\sin(2z)e} _{z} and be bounded by the following$$\backslash quad\; \backslash quad$$

The corresponding double contour integral would be set up as such:

We now evaluate

\nabla ⋅ F . Meanwhile, set up the corresponding triple integral:

Example 2

F = u ^{-3} _{y + z} e _{z} , and remark that there are 4 parameters in this case. Let this vector field be bounded by the following:$$\backslash quad\; \backslash quad\; \backslash quad$$

To evaluate this, we must utilize the divergence theorem as stated before, and we must evaluate

\nabla ⋅ F . Let dV = dx dy dz du

Thus, we can evaluate a contour integral with

n =4 . We can use the same method to evaluate contour integrals for any vector field with n> 4 as well.

Integral representation

An integral representation of a function is an expression of the function involving a contour integral. Various integral representations are known for many special functions. Integral representations can be important for theoretical reasons, e.g. giving analytic continuation or functional equations, or sometimes for numerical evaluations.

For example, the original definition of the Riemann zeta function via a Dirichlet series,$$\backslash zeta(s)\; =\; \backslash sum\_^\backslash infty\backslash frac,$$

is valid only for . But$$\backslash zeta(s)\; =\; -\; \backslash frac\; \backslash int\_H\backslash frac\; dt,$$

where the integration is done over the Hankel contour, is valid for all complex s not equal to 1.

See also

• Residue (complex analysis)
• Cauchy principal value
• Poisson integral
• Pochhammer contour

Further reading

• • Jean Jacquelin, Marko Riedel, Branche univalente, Les-Mathematiques.net, in French.
• Marko Riedel et al., Problème d'intégrale, Les-Mathematiques.net, in French.
• Marko Riedel et al., Integral by residue, math.stackexchange.com.
• W W L Chen, Introduction to Complex Analysis
• Various authors, sin límites ni cotas, es.ciencia.matematicas, in Spanish.

Notes and References

1. Book: Stalker, John . Complex Analysis: Fundamentals of the Classical Theory of Functions . 77 . 0-8176-4038-X . Springer . 1998.
2. Book: Complex Analysis . Joseph . Bak . Donald J. . Newman . Chapters 11 & 12 . 130–156 . https://books.google.com/books?id=JX2YSgfZwbYC&q=%22contour+integral%22&pg=PA130 . 0-387-94756-6 . 1997 . Springer.
3. Book: Krantz, Steven George . Handbook of Complex Variables . Chapter 2 . https://books.google.com/books?id=aYU2AdF_0dIC&q=Calculus++Residues+inauthor:krantz&pg=PT13 . 0-8176-4011-8 . 1999 . Springer .
4. Book: The Cauchy Method of Residues: Theory and Applications . Dragoslav S. . Mitrinović . Jovan D. . Kečkić . 90-277-1623-4 . 1984 . Springer . Chapter 2 .
5. Book: Chapter 5 . The Cauchy Method of Residues: Theory and Applications. Dragoslav S. . Mitrinović . Jovan D. . Kečkić . 1984 . Springer . 90-277-1623-4 .
6. Book: Fundamentals of Complex Analysis with Applications to Engineering, Science, and Mathematics . 3rd . Chapter 4 . Edward B. . Saff . Arthur David . Snider . 2003 . Prentice Hall . 0-1390-7874-6.