# Lituus

A **lituus** is a spiral in which the angle is inversely proportional to the square of the radius (as expressed in polar coordinates).

- [math]r^2\theta = k \,[/math]

### The JavaScript code to produce this picture

```
<jsxgraph width="500" height="500" box="box1">
var b1 = JXG.JSXGraph.initBoard('box1', {axis:true,originX: 250, originY: 250, unitX: 25, unitY: 25});
var k = b1.createElement('slider', [[1,8],[5,8],[0,1,4]]);
var c = b1.createElement('curve', [function(phi){return Math.sqrt(k.Value()/phi); }, [0, 0],0, 8*Math.PI],
{curveType:'polar', strokewidth:4});
</jsxgraph>
```

### Other curves

A **rose** or **rhodonea curve** is a sinusoid plotted in polar coordinates. Up to similarity, these curves can all be expressed by a polar equation of the form

- [math] \!\,r=\cos(k\theta).[/math]

If k is an integer, the curve will be rose shaped with

* 2k petals if k is even, and * k petals if k is odd.

When k is even, the entire graph of the rose will be traced out exactly once when the value of θ changes from 0 to 2π. When k is odd, this will happen on the interval between 0 and π. (More generally, this will happen on any interval of length [math]2\pi[/math] for [math]k[/math] even, and [math]\pi[/math] for [math]k[/math] odd.)

The **quadrifolium** is a type of rose curve with n=2. It has polar equation:

- [math] r = \cos(2\theta), \,[/math]

with corresponding algebraic equation

- [math] (x^2+y^2)^3 = (x^2-y^2)^2. \, [/math]