# Extensive Definition

In the
differential geometry of curves, the evolute of a curve is the locus
of all its centers of
curvature. Equivalently, it is the envelope
of the normals to
a curve. The original curve is an involute of its evolute.
(Compare Media:Evolute2.gif
and Media:Involute.gif)

## History

Apollonius (c. 200 BC) discussed evolutes in Book V of his Conics. However, Huygens is sometimes credited with being the first to study them (1673).## Equations

Let (x, y) = (x(t), y(t)) be a parametrically defined plane curve. Let R = 1/\kappa be the radius of curvature and \phi be the tangential angle. Then the center of curvature at (x, y) is given by (x - R \sin \phi, y + R \cos \phi) and we may take (X, Y) = (x - R \sin \phi, y + R \cos \phi) as parametric equations for the evolute. We have (\cos \phi, \sin \phi) = \frac and R = 1/\kappa = \frac, so we may eliminate R and \phi to obtain:(X, Y)= (x-y'\frac, y+x'\frac)

If the curve (x, y) is parametrized by arc length s
(i.e. (x, y) = (x(s), y(s)) where |(x', y')|=1; see
natural parametrization) then this simplifies to: (X, Y)=
(x+\frac, y+\frac).

## Properties

Differentiating (X, Y) = (x - R \sin \phi, y + R \cos \phi) with respect to s we obtain: \frac (X, Y) = (\frac - R \cos \phi \frac - \frac\sin \phi, \frac - R \sin \phi \frac + \frac\cos \phi). \frac = \cos \phi, \frac = \sin \phi and \frac = \kappa = 1/R, so this simplifies to \frac (X, Y) = (-\frac\sin \phi, \frac\cos \phi) = \frac(-\sin \phi,\cos \phi). Which has magnitude |\frac| and direction \phi \pm \pi/2. This has the following implications:- The tangential angle of the evolute is \phi \pm \pi/2. (The sign of \pm \pi/2 is determined by the sign of \frac.)
- The tangent to the evolute is normal to the original curve. A curve is the envelope of its tangents so the evolute is also the envelope of the lines normal to the curve.
- The arclength along the curve (X, Y) from (X(s_1), Y(s_1)) to (X(s_2), Y(s_2)) is given by

- The original curve is an involute of the evolute.

If \phi can be solved as a function of R, say
\phi = g(R), then the Whewell
equation for the evolute is \Phi = g(R) + \pi/2, where \Phi is
the tangential angle of the evolute and we take R as arclength
along the evolute. From this we can derive the Cesàro
equation as \Kappa = g'(R), where \Kappa is the curvature of the
evolute.

### Relationship between a curve and its evolute

By the above discussion, the derivative of (X, Y)
vanishes when \frac = 0, so the evolute will have a cusp
when the curve has a vertex,
that is when the curvature has a local maximum or minimum. At a
point of inflection of the original curve the radius of curvature
becomes infinite and so (X, Y) will become infinite, often this
will result in the evolute having an asymptote. Similarly, when the
original curve has a cusp where the radius of curvature is 0 then
the evolute will touch the the original curve.

This can be seen in the figure to the right, the
blue curve is the evolute of all the other curves. The cusp in the
blue curve corresponds to a vertex in the other curves. The cusps
in the green curve are on the evolute. Curves with the same evolute
are parallel.

## Radial of a curve

A curve with a similar definition is the Radial of a given curve. For each point on the curve take the vector from the point to the center of curvature and translate it so that it begins at the origin. Then the locus of points at the end of such vectors is called the Radial of the curve. The equation for the radial is obtained by removing the x and y terms from the equation of the evolute. Ths produces (X, Y)= (- R \sin \phi, R \cos \phi) or (X, Y)= (-y'\frac, x'\frac).## Examples

- The evolute of a parabola is a semicubical parabola. The cusp of the latter curve is the center of curvature of the parabola at its vertex.

- The evolute of a Logarithmic spiral is a congruent spiral.

- The evolute of a cycloid is a similar cycloid.

Yates, R. C.: A Handbook on Curves and Their
Properties, J. W. Edwards (1952), "Evolutes." pp. 86ff

evolutes in Czech: Evoluta

evolutes in German: Evolute

evolutes in Spanish: Evoluta

evolutes in French: Développée

evolutes in Italian: Evoluta

evolutes in Dutch: Evolute

evolutes in Polish: Ewoluta

evolutes in Russian: Эволюта

evolutes in Slovak: Evolúta

evolutes in Finnish: Evoluutta