Introduction to Step Control

Even with Newton's method where the local model is based on the actual Hessian, unless you are close to a root or minimum, the model step may not bring you any closer to the solution. A simple example is given by the following problem.

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This shows a simple example for root finding with step control disabled where the iteration alternates between two points and does not converge. Note: On some platforms, you may see convergence. This is due to slight variations in machine-number arithmetic, which may be sufficient to break the oscillation.
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This shows the same example problem with step control enabled. Since the first evaluation point has not reduced the size of the function, the line search restricts the step and so the iteration converges to the solution.
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A good step-size control algorithm will prevent repetition or escape from areas near roots or minima from happening. At the same time, however, when steps based on the model function are appropriate, the step-size control algorithm should not restrict them, otherwise the convergence rate of the algorithm would be compromised. Two commonly used step-size control algorithms are line search and trust region methods. In a line search method, the model function gives a step direction, and a search is done along that direction to find an adequate point that will lead to convergence. In a trust region method, a distance in which the model function will be trusted is updated at each step. If the model step lies within that distance, it is used; otherwise, an approximate minimum for the model function on the boundary of the trust region is used. Generally the trust region methods are more robust, but they require more numerical linear algebra.

Both step control methods were developed originally with minimization in mind. However, they apply well to finding roots for nonlinear equations when used with a merit function. In the Wolfram Language, the 2-norm merit function is used.

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