improve_tcg : bool, optional
    If True, a solution generated by the truncated conjugate gradient
    method that is on the boundary of the trust region is improved by
    moving around the trust-region boundary on the two-dimensional space
    spanned by the solution and the gradient of the quadratic function at
    the solution (default is True).

Notes
-----
This function implements Algorithm 6.3 of [1]_. It is assumed that the
origin is feasible with respect to the bound and linear constraints, and
that `delta` is finite and positive.

References
----------
.. [1] T. M. Ragonneau. *Model-Based Derivative-Free Optimization Methods
   and Software*. PhD thesis, Department of Applied Mathematics, The Hong
   Kong Polytechnic University, Hong Kong, China, 2022. URL:
   https://theses.lib.polyu.edu.hk/handle/200/12294.
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