# Definition:Cauchy Principal Value

## Contents

## Definition

The **Cauchy Principal Value** is an extension of the concept of an improper integral when the latter might not exist.

### Real Integrals

Let $f: \R \to \R$ be a real function which is piecewise continuous everywhere.

Then the **Cauchy Principal Value of $\displaystyle \int f$** is defined as:

- $\displaystyle \operatorname {PV}\int_{-\infty}^{+\infty} f \left({t}\right) \rd t := \lim_{R \mathop \to +\infty} \int_{-R}^R f \left({t}\right) \rd t$

where $\displaystyle \int_{-R}^R f \left({t}\right) \rd t$ is a Riemann Integral.

### Complex Integrals

Let $f: \R \to \C$ be a bounded complex function.

Then the **Cauchy Principal Value of $\displaystyle \int f$** is defined as:

- $\displaystyle \operatorname {PV} \int_{-\infty}^{+\infty} f \left({t}\right) \rd t := \lim_{R \mathop \to +\infty} \int_{-R}^R f \left({t}\right) \rd t$

where $\displaystyle \int_{-R}^R f \left({t}\right) \rd t$ is a complex Riemann integral.

### Contour Integrals

Let $C$ be a contour defined by a directed smooth curve.

Let $C$ be parameterized by the smooth path $\phi: \left[{-R \,.\,.\, R}\right] \to \C$, where $R > 0$.

Let $f: \operatorname{Im} \left({C}\right) \to \C$ be a continuous complex function, where $\operatorname{Im} \left({C}\right)$ denotes the image of $C$.

Then the **Cauchy Principal Value of $\displaystyle \int f$** is defined as:

- $\displaystyle \operatorname {PV} \int_C f \left({z}\right) \rd z = \displaystyle \operatorname {PV} \int_{\phi \left({-\infty}\right)}^{\phi \left({+\infty}\right)} f \left({z}\right) \rd z := \lim_{R \mathop \to +\infty} \int_{-R}^R f \left({\phi \left({t}\right)}\right)\phi' \left({t}\right) \rd t$

where $\displaystyle \int_{-R}^R f \left({\phi \left({t}\right)}\right) \phi'\left({t}\right) \rd t$ is a Complex Riemann Integral defining a Contour Integral.

## Also denoted as

Variants of the letters $\text P$ and $\text V$ can often be seen, such as:

- $\displaystyle \operatorname{P.V.} \int$

- $\displaystyle \operatorname{p.v.} \int$

- $\displaystyle PV \int$

and so on.

## Source of Name

This entry was named for Augustin Louis Cauchy.

## Sources

- 2004: James Ward Brown and Ruel V. Churchill:
*Complex Variables and Applications*(7th ed.): $\S 7$