.\" -*- nroff -*- generated from .Rd format
.BG
.FN varConstPower
.TL
Constant Plus Power Variance Function
.DN
This function is a constructor for the `varConstPower' class,
representing a constant plus power variance function
structure. Letting v denote the variance covariate and
s2(v) denote the variance function evaluated at
v, the constant plus power variance function is defined as
s2(v) = (t1 + |v|^t2)^2,
where t1, t2 are the variance  
function coefficients. When a grouping factor is present, different
t1, t2 are used for each factor level.
.CS
varConstPower(const, power, form, fixed)
.OA
.AG const, power
optional numeric vectors, or lists of numeric
values, with, respectively, the coefficients for the constant 
and the power terms. Both arguments must have length one, unless a
grouping factor is specified in `form'. If either argument has
length greater than one, it must have names which identify its
elements to the levels of the grouping factor defined in
`form'. If a grouping factor is present in 
`form' and the argument has length one, its value will be
assigned to all grouping levels. Only positive values are allowed
for `const'. Default is `numeric(0)', which
results in a vector of zeros of appropriate length being assigned to
the coefficients when `object' is initialized (corresponding
to constant variance equal to one).
.AG form
an optional one-sided formula of the form `~ v', or `~ v | g',
specifying a variance covariate `v' and, optionally, a grouping factor
`g' for the coefficients. The variance covariate must evaluate to a
numeric vector and may involve expressions using `"."', representing a
fitted model object from which fitted values (`fitted(.)') and
residuals (`resid(.)') can be extracted (this allows the variance
covariate to be updated during the optimization of an objective
function). When a grouping factor is present in `form', a different
coefficient value is used for each of its levels. Several grouping
variables may be simultaneously specified, separated by the `*'
operator, like in `~ v | g1 * g2 * g3'. In this case, the levels of
each grouping variable are pasted together and the resulting factor is
used to group the observations. Defaults to `~ fitted(.)'
representing a variance covariate given by the fitted values of a
fitted model object and no grouping factor.
.AG fixed
an optional list with components `const' and/or
`power', consisting of numeric vectors, or lists of numeric
values, specifying the values at which some or all of the
coefficients in the variance function should be fixed. If a grouping
factor is specified in `form', the components of `fixed'
must have names identifying which coefficients are to be
fixed. Coefficients included in `fixed' are not allowed to vary
during the optimization of an objective function. Defaults to
`NULL', corresponding to no fixed coefficients.
.RT
a `varConstPower' object representing a constant plus power
variance function structure, also inheriting from class
`varFunc'.

.SA
`varWeights.varFunc', `coef.varConstPower'
.EX
vf1 <- varConstPower(1.2, 0.2, form = ~age|Sex)
.KW models
.WR
