## Create a Periodic Interpolation Spline

## Usage

    periodicSpline(obj1, obj2, knots, period, ord)

## Arguments

`obj1` | Either a numeric vector of `x` values or a formula.
`obj2` | If `obj1` is numeric this should be a numeric vector of the same length. If `obj1` is a formula this can be an optional data frame in which to evaluate the names in the formula.
`knots` | An optional numeric vector of knot positions.
`period` | A positive numeric value giving the period for the periodic spline. Defaults to `2 * pi`.
`ord` | A positive integer giving the order of the spline. Defaults to 4. See `?splineOrder` for a definition of the order of a spline.

## Description

Create a periodic interpolation spline, either from `x` and `y` vectors, or from a formula/data.frame combination.

## Value

An object that inherits from class `spline`. The object can be in the B-spline representation, in which case it will be a `pbSpline` object, or in the piecewise polynomial representation (a `ppolySpline` object).

## Author(s)

Douglas Bates and Bill Venables

## See Also

`splineKnots`, `interpSpline`, `periodic.spline`

## Examples

    xx <- seq( -pi, pi, len = 16 )
    yy <- sin( xx )
    frm <- data.frame( xx, yy )
    pispl <- periodicSpline( xx, yy, period = 2 * pi )
    pispl2 <- periodicSpline( yy ~ xx, data = frm, period = 2 * pi )
    # pispl and pispl2 should be the same
    plot( predict( pispl, seq(-3*pi, 3*pi, len = 101) ) )
    plot( pispl )

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