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lines 5-85 of file: src/fix_likelihood.xrst
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{xrst_begin fix_likelihood}

User Defined Fixed Likelihood Function
######################################

Syntax
******
*vec* = *mixed_object* . ``fix_likelihood`` ( *fixed_vec* )

mixed_object
************
We use :ref:`derived_ctor@mixed_object`
to denote an object of a class that is
derived from the ``cppad_mixed`` base class.

a1_double
*********
see :ref:`typedef@Scalar Types@a1_double` .

Virtual Function
****************
The following virtual function may be implemented in the derived class:

| |tab| ``virtual CppAD::vector<a1_double> fix_likelihood`` (
| |tab| |tab| ``const CppAD::vector<a1_double>&`` *fixed_vec*
| |tab| )

fixed_vec
*********
This argument has prototype

    ``const CppAD::vector<a1_double>&`` *fixed_vec*

It contains a value for the
:ref:`fixed effects<problem@Notation@Fixed Effects, theta>` vector.

vec
***
This result has prototype

    ``CppAD::vector<`` *a1_double* > *vec*

It is a
:ref:`problem@Negative Log-Density Vector`
corresponding to the fixed likelihood
:ref:`g(theta)<theory@Fixed Likelihood, g(theta)>` where

.. math::

    g( \theta ) = - \log [ \B{p} ( z | \theta ) \B{p} ( \theta ) ]

constant
========
Adding or subtracting a constant to *vec* [0] ,
that does not depend on the fixed effects :math:`\theta`,
does not affect the optimal estimates for the fixed or random effects.

Default
=======
The base class definition (default) for ``fix_likelihood``
returns an empty vector;
i.e., *vec* . ``size`` () == 0 .
This corresponds to the case where:

#. All of the data depends on the
   random effects; i.e., the data vector
   :ref:`z<problem@Notation@Data, y, z>` is empty.
#. There is no prior
   :ref:`p(theta)<problem@Notation@Fixed Prior Density, p(theta)>`
   for the fixed effects.

{xrst_toc_hidden
    example/user/fix_likelihood.cpp
}
Example
*******
The file :ref:`fix_likelihood.cpp-name` contains an example and test
of defining this virtual function.
It returns true, if the test passes, and false otherwise.

{xrst_end fix_likelihood}