# Documentation

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# `RationalExpression`

Rational expression in a rootlocus plot

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## Graphics Primitives

ObjectsRationalExpression Default Values
`plot::Rootlocus`

## Description

`RationalExpression` is the internal name of the expression whose roots are depicted by `plot::Rootlocus`.

A rootlocus plot depicts the roots of a rational function ```p(z, u)``` in the complex plane, depending on a parameter `u`. The expression `p(z, u)` is stored as the attribute `RationalExpression` in the rootlocus object.

Usually, a user will have no need to access the attribute `RationalExpression`, since it is set by `plot::Rootlocus` directly.

## Examples

### Example 1

We define a rootlocus plot:

```r:= plot::Rootlocus(z^2 - 2*u*z + 0.81, u = -1..1): plot(r)```

The function z2 - 2 uz + 0.81 is stored as the attribute `r::RationalExpression` in the object `r`:

`r::RationalExpression`

We can redefine this attribute:

```r::RationalExpression:= z^3 - 2*u*z + 0.81: plot(r)```