After the initialization function
nag_opt_handle_init (e04rac) has been called,
nag_opt_handle_set_simplebounds (e04rhc) may be used to define the variable bounds
of the problem unless the bounds have already been defined. This will typically be used for problems, such as linear programming (LP)
quadratic programming (QP)
nonlinear programming (NLP)
or linear semidefinite programming (SDP)
where
and
are
-dimensional vectors. Note that upper and lower bounds are specified for all the variables. This form allows full generality in specifying various types of constraint. In particular, the
th variable may be fixed by setting
. If certain bounds are not present, the associated elements of
or
may be set to special values that are treated as
or
. See the description of the optional parameter
which is common among all solvers in the suite. Its value is denoted as
further in this text. Note that the bounds are interpreted based on its value at the time of calling this function and any later alterations to
will not affect these constraints.
Not applicable.
Internal changes have been made to this function as follows:
- At Mark 26.1: The limitation to specifying model fixed variables ( for some ) where such input returns NE_BOUND has been lifted. The only solver which cannot handle fixed variables (nag_opt_handle_solve_pennon (e04svc)) will report an error on such a model directly.
For details of all known issues which have been reported for the NAG Library please refer to the
Known Issues list.
There is a vast number of problems which can be reformulated as SDP. This example follows
Candes and Recht (2009) to show how a rank minimization problem can be approximated by SDP. In addition, it demonstrates how to work with the monitor mode of
nag_opt_handle_solve_pennon (e04svc).
Rank minimization is in general NP-hard but it can be approximated by a heuristic, minimizing the nuclear norm of the matrix. The nuclear norm of a matrix is the sum of its singular values. A rank deficient matrix must have (several) zero singular values. Given the fact that the singular values are always non-negative, a minimization of the nuclear norm has the same effect as norm in compress sensing, i.e., it encourages many singular values to be zero and thus it can be considered as a heuristic for the original rank minimization problem.
Let
denote the partially filled in
matrix with the valid responses on
positions. We are looking for
of the same size so that the valid responses are unchanged and the nuclear norm (denoted here as
) is minimal.
This is equivalent to
which is the linear semidefinite problem solved in this example, see
Candes and Recht (2009) and the references therein for details.
This example has
respondents and
answers. The obtained answers are
where
denotes missing entries (
is used instead in the data file). The obtained matrix has rank
and it is shown below printed to
-digit accuracy:
The example also turns on monitor mode of
nag_opt_handle_solve_pennon (e04svc), there is a time limit introduced for the solver which is being checked at the end of every outer iteration. If the time limit is reached, the function is stopped by setting
within the monitor step.
See also
Section 10 in
nag_opt_handle_init (e04rac) for links to further examples in the suite.