

   FFiixxeedd VVaarriiaannccee FFuunnccttiioonn

        varFixed(value)

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

      value: a one-sided formula of the form `~ v' specifying a
             variance covariate `v'. Grouping factors are
             ignored.

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

        This function is a constructor for the `varFixed'
        class, representing a variance function with fixed
        variances. Letting v denote the variance covariate
        defined in `value', the variance function s2(v) for
        this class is s2(v)=|v|. The variance covariate v is
        evaluated once at initialization and remains fixed
        thereafter. No coefficients are required to represent
        this variance function.

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

        a `varFixed' object representing a fixed variance func-
        tion structure, also inheriting from class `varFunc'.

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

        Jose Pinheiro and Douglas Bates

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

        `varWeights.varFunc', `varFunc'

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

        library(lme)
        vf1 <- varFixed(~age)

