# Cissoid

In geometry, a cissoid is a curve generated from two given curves C1, C2 and a point O (the pole). Let L be a variable line passing through O and intersecting C1 at P1 and C2 at P2. Let P be the point on L so that OP = P1P2. (There are actually two such points but P is chosen so that P is in the same direction from O as P2 is from P1.) Then the locus of such points P is defined to be the cissoid of the curves C1, C2 relative to O.

Slightly different but essentially equivalent definitions are used by different authors. For example, P may be defined to be the point so that OP = OP1 + OP2. This is equivalent to the other definition if C1 is replaced by its reflection through O. Or P may be defined as the midpoint of P1 and P2; this produces the curve generated by the previous curve scaled by a factor of 1/2.

The word "cissoid" comes from the Greek kissoeidēs "ivy shaped" from kissos "ivy" and -oeidēs "having the likeness of".

## Equations

If C1 and C2 are given in polar coordinates by $r=f_1(\theta)$ and $r=f_2(\theta)$ respectively, then the equation $r=f_2(\theta)-f_1(\theta)$ describes the cissoid of C1 and C2 relative to the origin. However, because a point may be represented in multiple ways in polar coordinates, there may be other branches of the cissoid which have a different equation. Specifically, C1 is also given by

$r=-f_1(\theta+\pi),\ r=-f_1(\theta-\pi),\ r=f_1(\theta+2\pi),\ r=f_1(\theta-2\pi),\ \dots$.

So the cissoid is actually the union of the curves given by the equations

$r=f_2(\theta)-f_1(\theta),\ r=f_2(\theta)+f_1(\theta+\pi),\ r=f_2(\theta)+f_1(\theta-\pi),\$
$r=f_2(\theta)-f_1(\theta+2\pi),\ r=f_2(\theta)-f_1(\theta-2\pi),\ \dots$.

It can be determined on an individual basis depending on the periods of f1 and f2, which of these equations can be eliminated due to duplication.

Ellipse $r=\frac{1}{2-\cos \theta}$ in red, with its two cissoid branches in black and blue (origin)

For example, let C1 and C2 both be the ellipse

$r=\frac{1}{2-\cos \theta}$.

The first branch of the cissoid is given by

$r=\frac{1}{2-\cos \theta}-\frac{1}{2-\cos \theta}=0$,

which is simply the origin. The ellipse is also given by

$r=\frac{-1}{2+\cos \theta}$,

so a second branch of the cissoid is given by

$r=\frac{1}{2-\cos \theta}+\frac{1}{2+\cos \theta}$

which is an oval shaped curve.

If each C1 and C2 are given by the parametric equations

$x = f_1(p),\ y = px$

and

$x = f_2(p),\ y = px$,

then the cissoid relative to the origin is given by

$x = f_2(p)-f_1(p),\ y = px$.

## Specific cases

When C1 is a circle with center O then the cissoid is conchoid of C2.

When C1 and C2 are parallel lines then the cissoid is a third line parallel to the given lines.

### Hyperbolas

Let C1 and C2 be two non-parallel lines and let O be the origin. Let the polar equations of C1 and C2 be

$r=\frac{a_1}{\cos (\theta-\alpha_1)}$

and

$r=\frac{a_2}{\cos (\theta-\alpha_2)}$.

By rotation through angle $(\alpha_1-\alpha2)/2$, we can assume that $\alpha_1 = \alpha,\ \alpha_2 = -\alpha$. Then the cissoid of C1 and C2 relative to the origin is given by

$r=\frac{a_2}{\cos (\theta+\alpha)} - \frac{a_1}{\cos (\theta-\alpha)}$
$=\frac{a_2\cos (\theta-\alpha)-a_1\cos (\theta+\alpha)}{\cos (\theta+\alpha)\cos (\theta-\alpha)}$
$=\frac{(a_2\cos\alpha-a_1\cos\alpha)\cos\theta-(a_2\sin\alpha+a_1\sin\alpha)\sin\theta} {\cos^2\alpha\ \cos^2\theta-\sin^2\alpha\ \sin^2\theta}$.

Combining constants gives

$r=\frac{b\cos\theta+c\sin\theta}{\cos^2\theta-m^2\sin^2\theta}$

which in Cartesian coordinates is

$x^2-m^2y^2=bx+cy$.

This is a hyperbola passing though the origin. So the cissoid of two non-parallel lines is a hyperbola containing the pole. A similar derivation show that, conversely, any hyperbola is the cissoid of two non-parallel lines relative to any point on it.

A cissoid of Zahradnik (name after Karel Zahradnik) is defined as the cissoid of a conic section and a line relative to any point on the conic. This is a broad family of rational cubic curves containing several well-known examples. Specifically:

$2x(x^2+y^2)=a(3x^2-y^2)$
is the cissoid of the circle $(x+a)^2+y^2 = a^2$ and the line $x={-{a \over 2}}$ relative to the origin.
$y^2(a+x) = x^2(a-x)$
is the cissoid of the circle $(x+a)^2+y^2 = a^2$ and the line $x=-a$ relative to the origin.
$x(x^2+y^2)+2ay^2=0$
is the cissoid of the circle $(x+a)^2+y^2 = a^2$ and the line $x=-2a$ relative to the origin. This is, in fact, the curve for which the family is named and some authors refer to this as simply as cissoid.
• The cissoid of the circle $(x+a)^2+y^2 = a^2$ and the line $x=ka$, where k is a parameter, is called a Conchoid of de Sluze. (These curves are not actually concoids.) This family includes the previous examples.
$x^3+y^3=3axy$
is the cissoid of the ellipse $x^2-xy+y^2 = -a(x+y)$ and the line $x+y=-a$ relative to the origin. To see this, note that the line can be written
$x=-\frac{a}{1+p},\ y=px$
and the ellipse can written
$x=-\frac{a(1+p)}{1-p+p^2},\ y=px$.
So the cissoid is given by
$x=-\frac{a}{1+p}+\frac{a(1+p)}{1-p+p^2} = \frac{3ap}{1+p^3},\ y=px$
which is a parametric form of the folium.