## What is trust region reflective algorithm?

‘trust-region-reflective’ requires you to provide a gradient, and allows only bounds or linear equality constraints, but not both. Within these limitations, the algorithm handles both large sparse problems and small dense problems efficiently. It is a large-scale algorithm; see Large-Scale vs. Medium-Scale Algorithms.

## What is the importance of Cauchy point in trust region methods?

Cauchy point is the minimizer of the sub-problem along the steepest descent direction within the trust-region as shown in the figure below.

**What is Cauchy point?**

The Cauchy point is the point lying on the gradient which minimises the quadratic model subject to the step being within the trust region. By iteratively finding the Cauchy point the local minimum can be found. The convergence of the technique is inefficient, being similar to that of the steepest descent algorithm.

**What is line search used for?**

Line search is an optimization algorithm for univariate or multivariate optimization. The algorithm requires an initial position in the search space and a direction along which to search.

### What is the main difference between the line search methods and trust region methods?

Traditional iterative methods for solving (1) are either line search method or trust region method. Line search method is based on searching a new iterative point along a descent direction at each iteration and trust region method is based on finding a new iterative point within a ball centered at the current iterate.

### What method does Fsolve use?

Equation Solving Definition fsolve attempts to solve a system of equations by minimizing the sum of squares of the components. If the sum of squares is zero, the system of equations is solved.

**How do you approximate Hessian?**

One method for approximating the Hessian matrix is to use difference approximations. Difference approximation methods exploit the fact that each column of the Hessian can be approximated by taking the difference between two instances of the gradient vector evaluated at two nearby points.

**What are line search methods?**

Line search method is an iterative approach to find a local minimum of a multidimensional nonlinear function using the function’s gradients. It computes a search direction and then finds an acceptable step length that satisfies certain standard conditions.

#### What are search methods in optimization?

Search methods for smooth optimization problems are based on the assumption that all functions of the problem are continuous and at least twice continuously differentiable. Also, accurate first-order derivatives of all the functions are available.

#### What is the difference between line search based descent algorithm and trust region based descent algorithm?

Line search method is based on searching a new iterative point along a descent direction at each iteration and trust region method is based on finding a new iterative point within a ball centered at the current iterate.

**What algorithm does Matlab Fsolve use?**

trust-region dogleg algorithm

By default fsolve chooses the trust-region dogleg algorithm. The algorithm is a variant of the Powell dogleg method described in [8]. It is similar in nature to the algorithm implemented in [7]. See Trust-Region-Dogleg Algorithm.

**Is there an open research direction for line search and trust region?**

In machine learning settings, it is uncommon to find line search or trust region methods — or let’s call it: an open research direction. Machine learning methods rely on the idea of stochastic gradients — even though the objective function is defined over a large set of data, only a small batch is used to estimate the gradients.

## How to solve trust-region sub-problem?

(Note that hessian or approximate hessian will be evaluated in dogleg method) The most widely used method for solving a trust-region sub-problem is by using the idea of conjugated gradient (CG) method for minimizing a quadratic function since CG guarantees convergence within a finite number of iterations for a quadratic programming.

## What is the trust-region of a function?

In most cases, the trust-region is defined as a spherical area of radius in which the trust-region subproblem lies. If we are using the quadratic model to approximate the original objective function, then our optimization problem is essentially reduced to solving a sequence of trust-region subporblems

**What is the trust region radius?**

In most cases, the trust-region is defined as a spherical area of radius in which the trust-region subproblem lies. Where is the trust region radius, is the gradient at current point and is the hessian (or a hessian approximation).