

   EExxttrraacctt MMaattrriixx oorr SSqquuaarree--RRoooott FFaaccttoorr ffrroomm aa ppddMMaatt OObbjjeecctt

        pdMatrix(object, fact, ...)

   AArrgguummeennttss::

     object: an object inheriting from class code{pdMat}, rep-
             resenting a positive definite matrix.

       fact: an optional logical value. If `TRUE', a square-
             root factor of the positive-definite matrix repre-
             sented by `object' is returned; else, if `FALSE',
             the positive-definite matrix is returned. Defaults
             to `FALSE'.

        ...: some methods for the generic function may require
             additional arguments.

   DDeessccrriippttiioonn::

        The positive-definite matrix represented by `object',
        or a square-root factor of it is obtained. Letting S
        denote a positive-definite matrix, a square-root factor
        of S is any square matrix L such that S = L'L. This
        function extracts S or L.

   VVaalluuee::

        if `fact' is `FALSE' the positive-definite matrix rep-
        resented by `object' is returned; else a square-root of
        the positive-definite matrix is returned.

   AAuutthhoorr((ss))::

        Jose Pinheiro and Douglas Bates

   SSeeee AAllssoo::

        `as.matrix.pdMat', `pdFactor', `corMatrix'

   EExxaammpplleess::

        library(lme)
        pd1 <- pdSymm(diag(1:4))
        pdMatrix(pd1)

