winding number

Winding numbers are a basic notion in algebraic topology, and play an important role in connection with analytic functions of a complex variable. Intuitively, given a closed curve $t\mapsto S(t)$ in an oriented Euclidean plane (such as the complex plane $ℂ$), and a point $p$ not in the image of $S$, the winding number (or index) of $S$ with respect to $p$ is the net number of times $S$ surrounds $p$. It is not altogether easy to make this notion rigorous.

Let us take $ℂ$ for the plane. We have a continuous mapping $S:[a,b]\to ℂ$ where $a$ and $b$ are some reals with and $S(a)=S(b)$. Denote by $\theta (t)$ the angle from the positive real axis to the ray from $z_0$ to $S(t)$. As $t$ moves from $a$ to $b$, we expect $\theta$ to increase or decrease by a multiple of $2\pi$, namely $2\omega \pi$ where $\omega$ is the winding number. One therefore thinks of using integration. And indeed, in the theory of functions of a complex variable, it is proved that the value

$$\frac{1}{2\pi i}{\int }_S\frac{dz}{z-z_0}$$

is an integer and has the expected properties of a winding number around $z_0$. To define the winding number in this way, we need to assume that the closed path $S$ is rectifiable (so that the path integral is defined). An equivalent condition is that the real and imaginary parts of the function $S$ are of bounded variation.

But if $S$ is any continuous mapping $[a,b]\to ℂ$ having $S(a)=S(b)$, the winding number is still definable, without any integration. We can break up the domain of $S$ into a finite number of intervals such that the image of $S$, on any of those intervals, is contained in a disc which does not contain $z_0$. Then $2\omega \pi$ emerges as a finite sum: the sum of the angles subtended at $z_0$ by the sides of a polygon.

Let $A$, $B$, and $C$ be any three distinct rays from $z_0$. The three sets

$$S^{-1}(A)\mathit{\hspace{1em}\hspace{1em}}{S}^{-1}(B)\mathit{\hspace{1em}\hspace{1em}}{S}^{-1}(C)$$

are closed in $[a,b]$, and they determine the winding number of $S$ around $z_0$. This result can provide an alternative definition of winding numbers in $ℂ$, and a definition in some other spaces also, but the details are rather subtle.

For one more variation on the theme, let $S$ be any topological space homeomorphic to a circle, and let $f:S\to S$ be any continuous mapping. Intuitively we expect that if a point $x$ travels once around $S$, the point $f(x)$ will travel around $S$ some integral number of times, say $n$ times. The notion can be made precise. Moreover, the number $n$ is determined by the three closed sets

$$f^{-1}(a)\mathit{\hspace{1em}\hspace{1em}}{f}^{-1}(b)\mathit{\hspace{1em}\hspace{1em}}{f}^{-1}(c)$$

where $a$, $b$, and $c$ are any three distinct points in $S$.

Title	winding number
Canonical name	WindingNumber
Date of creation	2013-03-22 12:56:06
Last modified on	2013-03-22 12:56:06
Owner	CWoo (3771)
Last modified by	CWoo (3771)
Numerical id	8
Author	CWoo (3771)
Entry type	Definition
Classification	msc 55M25
Classification	msc 30A99
Synonym	index