Blurb::
Specification group for trust region model management
Description::
The \c trust_region optional group specification can be used to
specify the initial size of the trust region (using \c initial_size)
relative to the total variable bounds, the minimum size of the trust
region (using \c minimum_size), the contraction factor for the trust
region size (using \c contraction_factor) used when the surrogate
model is performing poorly, and the expansion factor for the trust
region size (using \c expansion_factor) used when the the surrogate
model is performing well. Two additional commands are the trust region
size contraction threshold (using \c contract_threshold) and the trust
region size expansion threshold (using \c expand_threshold). These two
commands are related to what is called the trust region ratio, which
is the actual decrease in the truth model divided by the predicted
decrease in the truth model in the current trust region. The command
\c contract_threshold sets the minimum acceptable value for the trust
region ratio, i.e., values below this threshold cause the trust region
to shrink for the next surrogate_based_local iteration. The command \c
expand_threshold determines the trust region value above which the
trust region will expand for the next surrogate_based_local iteration.

Each of these specifications are \c REAL scalars, with the exception of
\c initial_size, which is a \c REALLIST.

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