backSpline Monotone Inverse Spline

Description

Create a monotone inverse of a monotone natural spline.

Usage

backSpline(object)

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.

Author(s)

Douglas Bates and Bill Venables

See Also

interpSpline

Examples

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 )