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% File nlme/man/pdMatrix.Rd
% Part of the nlme package for R
% Distributed under GPL 2 or later: see nlme/LICENCE.note
\name{pdMatrix}
\title{Extract Matrix or Square-Root Factor from a pdMat Object}
\usage{
pdMatrix(object, factor)
}
\alias{pdMatrix}
\alias{pdMatrix.pdBlocked}
\alias{pdMatrix.pdCompSymm}
\alias{pdMatrix.pdDiag}
\alias{pdMatrix.pdIdent}
\alias{pdMatrix.pdMat}
\alias{pdMatrix.pdSymm}
\alias{pdMatrix.pdNatural}
\arguments{
\item{object}{an object inheriting from class \code{pdMat}, representing
a positive definite matrix.}
\item{factor}{an optional logical value. If \code{TRUE}, a square-root
factor of the positive-definite matrix represented by \code{object}
is returned; else, if \code{FALSE}, the positive-definite matrix is
returned. Defaults to \code{FALSE}.}
}
\description{
The positive-definite matrix represented by \code{object}, or a
square-root factor of it is obtained. Letting \eqn{\Sigma}{S} denote a
positive-definite matrix, a square-root factor of \eqn{\Sigma}{S} is
any square matrix \eqn{L}{L} such that \eqn{\Sigma = L'L}{S =
L'L}. This function extracts \eqn{\Sigma}{S} or \eqn{L}.
}
\value{
if \code{factor} is \code{FALSE} the positive-definite matrix
represented by \code{object} is returned; else a square-root of the
positive-definite matrix is returned.
}
\references{
Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models
in S and S-PLUS", Springer, esp. p. 162.
}
\author{José Pinheiro and Douglas Bates \email{bates@stat.wisc.edu}}
\seealso{\code{\link{as.matrix.pdMat}},
\code{\link{pdClasses}},
\code{\link{pdFactor}},
\code{\link{pdMat}},
\code{\link{pdMatrix.reStruct}},
\code{\link{corMatrix}}
}
\examples{
pd1 <- pdSymm(diag(1:4))
pdMatrix(pd1)
}
\keyword{models}
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