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Globally solving the trust region subproblem using simple first-order methods. (English) Zbl 1455.90104
The authors introduce a family of first-order conic methods (FOCM) for solving the trust region subproblem $\left\{ \begin{array}{l} \mathrm{ minimize }\; q(x) := x^T A x - 2b^Tx \\ \mathrm{ s.t.}\\ x \in B := \{v \in \mathbb{R}^n \mid \|v\|^2 \leq 1\}, \end{array} \right.\tag{TRS}$ where $$q$$ is a quadratic (not necessarily convex) function given by a symmetric matrix $$A \in \mathbb{R}^{n \times n}$$ and $$b \in \mathbb{R}$$, while $$B$$ is the Euclidean closed unit ball. It is shown that any method within the family FOCM (in particular, the projected and conditional gradient methods) produces a sequence that converges to a stationary point of the problem TRS. The convergence to an optimal solution is proven in both the “easy case” and the “hard case” under suitable assumptions.

##### MSC:
 90C06 Large-scale problems in mathematical programming 90C26 Nonconvex programming, global optimization 90C46 Optimality conditions and duality in mathematical programming
GQTPAR; HSL-VF05
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