pdIdent                 package:nlme                 R Documentation

Multiple of the Identity Positive-Definite Matrix

Description:

     This function is a constructor for the 'pdIdent' class,
     representing a multiple of the identity positive-definite matrix. 
     The matrix associated with 'object' is represented by 1
     unrestricted parameter, given by the logarithm of the square-root
     of the diagonal value. When 'value' is  'numeric(0)', an
     uninitialized 'pdMat' object, a one-sided formula, or a vector of
     character strings, 'object' is returned as an uninitialized
     'pdIdent' object (with just some of its attributes and its class
     defined) and needs to have its coefficients assigned later,
     generally using the 'coef' or 'matrix' replacement functions. If
     'value' is an initialized 'pdMat' object, 'object' will be
     constructed from 'as.matrix(value)'. Finally, if 'value' is a
     numeric value, it is assumed to represent the unrestricted
     coefficient of the underlying positive-definite  matrix.

Usage:

     pdIdent(value, form, nam, data)

Arguments:

   value: an optional initialization value, which can be any of the
          following: a 'pdMat' object, a positive-definite matrix, a
          one-sided linear formula (with variables separated by '+'), a
          vector of character strings, or a numeric value. 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 in the
          formulas. Defaults to the parent frame from which the
          function was called.

Value:

     a 'pdIdent' object representing a multiple of the identity
     positive-definite matrix, also inheriting from class 'pdMat'.

Author(s):

     Jose Pinheiro Jose.Pinheiro@pharma.novartis.com and Douglas Bates
     bates@stat.wisc.edu

References:

     Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models in S
     and S-PLUS", Springer.

See Also:

     'as.matrix.pdMat', 'coef.pdMat', 'pdClasses', 'matrix<-.pdMat'

Examples:

     pd1 <- pdIdent(4 * diag(3), nam = c("A","B","C"))
     pd1

