# Cauchy principal value

This article is about a method for assigning values to improper integrals. For the values of a complex function associated with a single branch, see Principal value. For the negative-power portion of a Laurent series, see Principal part.

In mathematics, the Cauchy principal value, named after Augustin Louis Cauchy, is a method for assigning values to certain improper integrals which would otherwise be undefined.

## Formulation

Depending on the type of singularity in the integrand f, the Cauchy principal value is defined as one of the following:

1) The finite number
$\lim_{\varepsilon\rightarrow 0+} \left[\int_a^{b-\varepsilon} f(x)\,\mathrm{d}x+\int_{b+\varepsilon}^c f(x)\,\mathrm{d}x\right]$
where b is a point at which the behavior of the function f is such that
$\int_a^b f(x)\,\mathrm{d}x=\pm\infty$ for any a < b and
$\int_b^c f(x)\,\mathrm{d}x=\mp\infty$ for any c > b
(see plus or minus for precise usage of notations ±, ∓).
2) The infinite number
$\lim_{a\rightarrow\infty}\int_{-a}^a f(x)\,\mathrm{d}x$
where $\int_{-\infty}^0 f(x)\,\mathrm{d}x=\pm\infty$
and $\int_0^\infty f(x)\,\mathrm{d}x=\mp\infty$.
In some cases it is necessary to deal simultaneously with singularities both at a finite number b and at infinity. This is usually done by a limit of the form
$\lim_{\varepsilon \rightarrow 0+} \left[\int_{b-\frac{1}{\varepsilon}}^{b-\varepsilon} f(x)\,\mathrm{d}x+\int_{b+\varepsilon}^{b+\frac{1}{\varepsilon}}f(x)\,\mathrm{d}x \right].$
3) In terms of contour integrals

of a complex-valued function f(z); z = x + iy, with a pole on the contour. The pole is enclosed with a circle of radius ε and the portion of the path outside this circle is denoted L(ε). Provided the function f(z) is integrable over L(ε) no matter how small ε becomes, then the Cauchy principal value is the limit:[1]

$\mathrm{P} \int_{L} f(z) \ \mathrm{d}z = \int_L^* f(z)\ \mathrm{d}z = \lim_{\varepsilon \to 0 } \int_{L( \varepsilon)} f(z)\ \mathrm{d}z,$
where two of the common notations for the Cauchy principal value appear on the left of this equation.

In the case of Lebesgue-integrable functions, that is, functions which are integrable in absolute value, these definitions coincide with the standard definition of the integral.

Principal value integrals play a central role in the discussion of Hilbert transforms [2]

## Distribution theory

Let ${C_{c}^{\infty}}(\mathbb{R})$ be the set of bump functions, i.e., the space of smooth functions with compact support on the real line $\mathbb{R}$. Then the map

$\operatorname{p.\!v.} \left( \frac{1}{x} \right) \,:\, {C_{c}^{\infty}}(\mathbb{R}) \to \mathbb{C}$

defined via the Cauchy principal value as

$\left[ \operatorname{p.\!v.} \left( \frac{1}{x} \right) \right](u) = \lim_{\varepsilon \to 0^{+}} \int_{\mathbb{R} \setminus [- \varepsilon;\varepsilon]} \frac{u(x)}{x} \, \mathrm{d} x = \int_{0}^{+ \infty} \frac{u(x) - u(- x)}{x} \, \mathrm{d} x \quad \text{for } u \in {C_{c}^{\infty}}(\mathbb{R})$

is a distribution. The map itself may sometimes be called the principal value (hence the notation p.v.). This distribution appears, for example, in the Fourier transform of the Heaviside step function.

### Well-definedness as a distribution

To prove the existence of the limit

$\int_{0}^{+ \infty} \frac{u(x) - u(- x)}{x} \, \mathrm{d} x$

for a Schwartz function $u(x)$, first observe that $\frac{u(x) - u(-x)}{x}$ is continuous on $[0, \infty)$, as

$\lim\limits_{x \searrow 0} u(x) - u(-x) = 0$ and hence
$\lim\limits_{x\searrow 0} \frac{u(x) - u(-x)}{x} = \lim\limits_{x\searrow 0} \frac{u'(x) + u'(-x)}{1} = 2u'(0),$

since $u'(x)$ is continuous and LHospitals rule applies.

Therefore $\int\limits_0^1 \frac{u(x) - u(-x)}{x} \, \mathrm dx$ exists and by applying the mean value theorem to $u(x) - u(-x)$, we get that

$\left| \int\limits_0^1 \frac{u(x) - u(-x)}{x} \,\mathrm dx \right| \leq \int\limits_0^1 \frac{|u(x)-u(-x)|}{x} \,\mathrm dx \leq \int\limits_0^1 \frac{2x}{x} \sup\limits_{x \in \mathbb R} |u'(x)| \,\mathrm dx \leq 2 \sup\limits_{x \in \mathbb R} |u'(x)|$.

As furthermore

$\left| \int\limits_1^\infty \frac {u(x) - u(-x)}{x} \,\mathrm dx \right| \leq 2 \sup\limits_{x\in\mathbb R} |x\cdot u(x)| \int\limits_1^\infty \frac 1{x^2} \,\mathrm dx = 2 \sup\limits_{x\in\mathbb R} |x\cdot u(x)|,$

we note that the map $\operatorname{p.\!v.} \left( \frac{1}{x} \right) \,:\, {C_{c}^{\infty}}(\mathbb{R}) \to \mathbb{C}$ is bounded by the usual seminorms for Schwartz functions $u$. Therefore this map defines, as it is obviously linear, a continuous functional on the Schwartz space and therefore a tempered distribution.

Note that the proof needs $u$ merely to be continuously differentiable in a neighbourhood of $0$ and $xu$ to be bounded towards infinity. The principal value therefore is defined on even weaker assumptuions such as $u$ integrable with compact support and differentiable at 0.

### More general definitions

The principal value is the inverse distribution of the function $x$ and is almost the only distribution with this property:

$x f = 1 \quad \Rightarrow \quad f = \operatorname{p.\!v.} \left( \frac{1}{x} \right) + K \delta,$

where $K$ is a constant and $\delta$ the Dirac distribution.

In a broader sense, the principal value can be defined for a wide class of singular integral kernels on the Euclidean space $\mathbb{R}^{n}$. If $K$ has an isolated singularity at the origin, but is an otherwise "nice" function, then the principal-value distribution is defined on compactly supported smooth functions by

$[\operatorname{p.\!v.} (K)](f) = \lim_{\varepsilon \to 0} \int_{\mathbb{R}^{n} \setminus B_{\varepsilon(0)}} f(x) K(x) \, \mathrm{d} x.$

Such a limit may not be well defined, or, being well-defined, it may not necessarily define a distribution. It is, however, well-defined if $K$ is a continuous homogeneous function of degree $-n$ whose integral over any sphere centered at the origin vanishes. This is the case, for instance, with the Riesz transforms.

## Examples

Consider the difference in values of two limits:

$\lim_{a\rightarrow 0+}\left(\int_{-1}^{-a}\frac{\mathrm{d}x}{x}+\int_a^1\frac{\mathrm{d}x}{x}\right)=0,$
$\lim_{a\rightarrow 0+}\left(\int_{-1}^{-2 a}\frac{\mathrm{d}x}{x}+\int_{a}^1\frac{\mathrm{d}x}{x}\right)=\ln 2.$

The former is the Cauchy principal value of the otherwise ill-defined expression

$\int_{-1}^1\frac{\mathrm{d}x}{x}{\ } \left(\mbox{which}\ \mbox{gives}\ -\infty+\infty\right).$

Similarly, we have

$\lim_{a\rightarrow\infty}\int_{-a}^a\frac{2x\,\mathrm{d}x}{x^2+1}=0,$

but

$\lim_{a\rightarrow\infty}\int_{-2a}^a\frac{2x\,\mathrm{d}x}{x^2+1}=-\ln 4.$

The former is the principal value of the otherwise ill-defined expression

$\int_{-\infty}^\infty\frac{2x\,\mathrm{d}x}{x^2+1}{\ } \left(\mbox{which}\ \mbox{gives}\ -\infty+\infty\right).$

## Nomenclature

The Cauchy principal value of a function $f$ can take on several nomenclatures, varying for different authors. Among these are:

$PV \int f(x)\,\mathrm{d}x,$
$\int_L^* f(z)\, \mathrm{d}z,$
$\int f(x)\,\mathrm{d}x,$
as well as $P,$ P.V., $\mathcal{P},$ $P_v,$ $(CPV),$ and V.P.

## References

1. ^ Ram P. Kanwal (1996). Linear Integral Equations: theory and technique (2nd Edition ed.). Boston: Birkhäuser. p. 191. ISBN 0-8176-3940-3.
2. ^ Frederick W. King (2009). Hilbert Transforms. Cambridge: Cambridge University Press. ISBN 978-0-521-88762-5.