# Lemniscate of Bernoulli

## History

Studied by Jacob Bernoulli .

## Description

Lemniscate of Bernoulli is a special case of Cassinian oval. That is, the locus of points P, such that distance[P,F1] * distance[P,F2] == (distance[F1,F2]/2)^2, where F1, F2 are fixed points called foci. It is analogous to the definition of ellipse, where sum of two distances is replace by product.

## Formula

- Parametric: Cos[t]/(1+Sin[t]^2) {1, Sin[t]}, 0 < t ≤ 2 Pi.
- Polar: r^2 == Cos[2 * θ].
- Cartesian: (x^2 + y^2)^2 == (x^2-y^2).

Foci are at {-1/Sqrt[2],0}, {1/Sqrt[2],0}

## Properties

### Lemniscate as a Cissoid

Lemniscate of Bernoulli can be generated as a cissoid of two circles.

Step by step description:

- Let there be a circle with radius r.
- Let there be a point O, r*Sqrt[2] distant from the center of the circle.
- Draw a line passing through O and the circle. Let the intersections be Q1, Q2.
- Let there be a vector with origin at O whose length is distance[Q1,Q2].
- The locus of the vector is one loop of the lemniscate of Bernoulli. The other loop is symmetric with respect to O.

### Relation to Rectangular Hyperbola

Its inversion and negative pedal with respect to its center is the rectangular hyperbola. It is also the envelope of circles with centers on a rectangular hyperbola and each circle passing the hyperbola's center.

### Slicing a Torus

Lemniscate of Bernoulli is the intersection of a plane tangent to the inner ring of a torus whose inner radius equals to its radius of generating circle. (See: Cassinian oval).

### Construction of Tangent and Normal

The normal of any point P on the curve makes a angle 2 theta with the radius vector and 3 theta with the polar axis. The tangent of inclination is 2 theta + Pi/2.

### Generation by Linkage

Lemniscate of Bernoulli can be generated by these linkages. On the left: AB == ND == OD == c, AO == AN == BD == c/Sqrt[2]. P and Q are midpoints of line OD and ND respectively. The point P traces half a lemniscate and half a cicle. Same with Q. On the right: AB == AC == a, CE == BE == EF == a/Sqrt[2]

### Trivia

The math symbol for infinity is shaped like a lemniscate. It was first used by John Wallis in 1655 in his De Sectionibus conicis (See: A history of mathematical notations By Florian Cajori. Buy at amazon)