

   PPoossiittiivvee--DDeeffiinniittee MMaattrriixx CCllaasssseess

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

        Standard classes of positive-definite matrices
        (`pdMat') structures  available in the `lme' library.

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

        Available standard classes:

     pdSymm: general positive-definite matrix, with no addi-
             tional structure

     pdDiag: diagonal

    pdIdent: multiple of an identity

   pdCompSymm: compound symmetry structure (constant diagonal
             and constant off-diagonal elements)

   pdBlocked: block-diagonal matrix, with diagonal blocks of
             any "atomic" `pdMat' class

   pdNatural: general positive-definite matrix in natural
             parametrization (i.e. parametrized in terms of
             standard deviations and correlations). The under-
             lying coefficients are not unrestricted, so this
             class should NOT be used for optimization.

   NNoottee::

        Users may define their own `pdMat' classes by specify-
        ing a `constructor' function and, at a minimum, methods
        for the functions `pdConstruct', `pdMatrix' and `coef'.
        For examples of these functions, see the methods for
        classes `pdSymm' and `pdDiag'.

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

        Jose Pinheiro and Douglas Bates

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

        `pdCompSymm', `pdDiag', `pdIdent', `pdNatural',
        `pdSymm'

