Winding number

related topics
{math, number, function}
{math, energy, light}

In mathematics, the winding number of a closed curve in the plane around a given point is an integer representing the total number of times that curve travels counterclockwise around the point. The winding number depends on the orientation of the curve, and is negative if the curve travels around the point clockwise.

Winding numbers are fundamental objects of study in algebraic topology, and they play an important role in vector calculus, complex analysis, geometric topology, differential geometry, and physics.

Contents

Intuitive description

Suppose we are given a closed, oriented curve in the xy plane. We can imagine the curve as the path of motion of some object, with the orientation indicating the direction in which the object moves. Then the winding number of the curve is equal to the total number of counterclockwise turns that the object makes around the origin.

When counting the total number of turns, counterclockwise motion counts as positive, while clockwise motion counts as negative. For example, if the object first circles the origin four times counterclockwise, and then circles the origin once clockwise, then the total winding number of the curve is three.

Using this scheme, a curve that does not travel around the origin at all has winding number zero, while a curve that travels clockwise around the origin has negative winding number. Therefore, the winding number of a curve may be any integer. The following pictures show curves with winding numbers between −2 and 3:

Formal definition

A curve in the xy plane can be defined by parametric equations:

If we think of the parameter t as time, then these equations specify the motion of an object in the plane between t = 0 and t = 1. The path of this motion is a curve as long as the functions x(t) and y(t) are continuous. This curve is closed as long as the position of the object is the same at t = 0 and t = 1.

Full article ▸

related documents
Divergence
Buckingham π theorem
Surface
Density matrix
Hamiltonian (quantum mechanics)
Hypercube
Gauss–Bonnet theorem
Area
Conformal map
Distance
N-body problem
Del
Torus
Kaluza–Klein theory
Nonlinear system
N-sphere
Legendre polynomials
Angle trisection
Non-Euclidean geometry
List of letters used in mathematics and science
Poisson process
Divergence theorem
Depth-first search
Fixed point combinator
Jules Richard
Existential quantification
Compactness theorem
Outer product
Chain complex
Free variables and bound variables