

   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

        pdMat(value, form, nam, data, pdClass)

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

      value: an optional initialization value, which can be any
             of the following: a code{pdMat} object, a posi-
             tive-definite matrix, a one-sided linear formula
             (with variables separated by `+'), a vector of
             character strings, or a numeric vector. Defaults
             to `numeric(0)', corresponding to an uninitialized
             object.

       form: an optional one-sided linear formula specifying
             the row/column names for the matrix represented by
             `object'. Because factors may be present in
             `form', the formula needs to be evaluated on a
             data.frame to resolve the names it defines. This
             argument is ignored when `value' is a one-sided
             formula. Defaults to `NULL'.

        nam: an optional vector of character strings specifying
             the row/column names for the matrix represented by
             object. It must have length equal to the dimension
             of the underlying positive-definite matrix and
             unreplicated elements. This argument is ignored
             when `value' is a vector of character strings.
             Defaults to `NULL'.

       data: an optional data frame in which to evaluate the
             variables named in `value' and `form'. It is used
             to obtain the levels for `factors', which affect
             the dimensions and the row/column names of the
             underlying matrix. If `NULL', no attempt is made
             to obtain information on `factors' appearing the
             random effects model. Defaults to parent frame
             from which the function was called.

    pdClass: an optional character string naming the `pdMat'
             class to be assigned to the returned object. This
             argument will only be used when `value' is not a
             `pdMat' object. Defaults to `"pdSymm"'.

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

        This function gives an alternative way of constructing
        an object inheriting from the `pdMat' class named in
        `pdClass', or from `data.class(object)' if `object'
        inherits from `pdMat',  and is mostly used internally
        in other functions. See the documentation on the prin-
        cipal constructor function, generally with the same
        name as the `pdMat' class of object.

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

        a `pdMat' object representing a positive-definite
        matrix, inheriting from the class named in `pdClass',
        or from `class(object)', if `object' inherits from
        `pdMat'.

   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'

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

        library(lme)
        pd1 <- pdMat(diag(1:4), pdClass = "pdDiag")
        pd1

