# Tractrix

## History

From Robert Yates:

Studied by Huygens in 1692 and later by Leibnitz, Johann Bernoulli, Liouvlle, and Beltrami. Also called Tractory and Equitangential Curve.

## Description

Tracktrix (equitangential curve, tractory) is a curve such that any tangent segment from the tangent point on the curve to the curve's asymptote have constant length.

Suppose a bicycle with front wheel at the {0,0} and back wheel at {0,1}. The front wheel is turned to head East. The track traced by the back wheel is the tractrix. Note that the tractrix's asymptote cuts its tangents into segments of equal length.

## Formula

Parametric: {Log[Sec[t] + Tan[t]] - Sin[t], Cos[t]}, -π/2 < t < π/2.

Cartesian: x == Log[(1 -Sqrt[1^2-y^2])/y] + Sqrt[1^2-y^2].

## Properties

### Orthogonal to Circles

The tractrix is orthogonal to a set of circles centered on the tractrix's asymptote, all having radius radius R. R is the same as the tractrix's constant length tangent segment.

### Tractrix and Catenary

The evolute of tractrix is catenary, conversely, the involute of catenary is tractrix.

### Pseudosphere

The surface of revolution of tractrix around its asymptote is called pseudosphere. Eugenio Beltrami in 1868 showed that pseudosphere provided a model for hyperbolic geometry. It is a surface of constant negative Gaussian curvature.