# backSpline: Monotone Inverse Spline

## Description

Create a monotone inverse of a monotone natural spline.

## Arguments

object

an object that inherits from class `nbSpline`

or
`npolySpline`

. That is, the object must represent a natural
interpolation spline but it can be either in the B-spline
representation or the piecewise polynomial one. The spline is
checked to see if it represents a monotone function.

## Value

An object of class `polySpline`

that contains the piecewise
polynomial representation of a function that has the appropriate
values and derivatives at the knot positions to be an inverse of the
spline represented by `object`

. Technically this object is not a
spline because the second derivative is not constrained to be
continuous at the knot positions. However, it is often a much better
approximation to the inverse than fitting an interpolation spline to
the y/x pairs.

## Examples

# NOT RUN {
require(graphics)
ispl <- interpSpline( women$height, women$weight )
bspl <- backSpline( ispl )
plot( bspl ) # plots over the range of the knots
points( women$weight, women$height )
# }