In computational mathematics, an **iterative method** is a mathematical procedure that uses an initial value to generate a sequence of improving approximate solutions for a class of problems, in which the *n*-th approximation is derived from the previous ones. A specific implementation of an iterative method, including the termination criteria, is an algorithm of the iterative method. An iterative method is called **convergent** if the corresponding sequence converges for given initial approximations. A mathematically rigorous convergence analysis of an iterative method is usually performed; however, heuristic-based iterative methods are also common.

- Attractive fixed points
- Linear systems
- Stationary iterative methods
- Krylov subspace methods
- Preconditioners
- History
- See also
- References
- External links

In contrast, **direct methods** attempt to solve the problem by a finite sequence of operations. In the absence of rounding errors, direct methods would deliver an exact solution (like solving a linear system of equations by Gaussian elimination). Iterative methods are often the only choice for nonlinear equations. However, iterative methods are often useful even for linear problems involving many variables (sometimes of the order of millions), where direct methods would be prohibitively expensive (and in some cases impossible) even with the best available computing power.^{ [1] }

If an equation can be put into the form *f*(*x*) = *x*, and a solution **x** is an attractive fixed point of the function *f*, then one may begin with a point *x*_{1} in the basin of attraction of **x**, and let *x*_{n+1} = *f*(*x*_{n}) for *n* ≥ 1, and the sequence {*x*_{n}}_{n ≥ 1} will converge to the solution **x**. Here *x*_{n} is the *n*th approximation or iteration of *x* and *x*_{n+1} is the next or *n* + 1 iteration of *x*. Alternately, superscripts in parentheses are often used in numerical methods, so as not to interfere with subscripts with other meanings. (For example, *x*^{(n+1)} = *f*(*x*^{(n)}).) If the function *f* is continuously differentiable, a sufficient condition for convergence is that the spectral radius of the derivative is strictly bounded by one in a neighborhood of the fixed point. If this condition holds at the fixed point, then a sufficiently small neighborhood (basin of attraction) must exist.

In the case of a system of linear equations, the two main classes of iterative methods are the **stationary iterative methods**, and the more general Krylov subspace methods.

Stationary iterative methods solve a linear system with an operator approximating the original one; and based on a measurement of the error in the result (the residual), form a "correction equation" for which this process is repeated. While these methods are simple to derive, implement, and analyze, convergence is only guaranteed for a limited class of matrices.

An *iterative method* is defined by

and for a given linear system with exact solution the *error* by

An iterative method is called *linear* if there exists a matrix such that

and this matrix is called the *iteration matrix*. An iterative method with a given iteration matrix is called *convergent* if the following holds

An important theorem states that for a given iterative method and its iteration matrix it is convergent if and only if its spectral radius is smaller than unity, that is,

The basic iterative methods work by splitting the matrix into

and here the matrix should be easily invertible. The iterative methods are now defined as

From this follows that the iteration matrix is given by

Basic examples of stationary iterative methods use a splitting of the matrix such as

where is only the diagonal part of , and is the strict lower triangular part of . Respectively, is the strict upper triangular part of .

- Richardson method:
- Jacobi method:
- Damped Jacobi method:
- Gauss–Seidel method:
- Successive over-relaxation method (SOR):
- Symmetric successive over-relaxation (SSOR):

Linear stationary iterative methods are also called relaxation methods.

Krylov subspace methods work by forming a basis of the sequence of successive matrix powers times the initial residual (the **Krylov sequence**). The approximations to the solution are then formed by minimizing the residual over the subspace formed. The prototypical method in this class is the conjugate gradient method (CG) which assumes that the system matrix is symmetric positive-definite. For symmetric (and possibly indefinite) one works with the minimal residual method (MINRES). In the case of non-symmetric matrices, methods such as the generalized minimal residual method (GMRES) and the biconjugate gradient method (BiCG) have been derived.

Since these methods form a basis, it is evident that the method converges in *N* iterations, where *N* is the system size. However, in the presence of rounding errors this statement does not hold; moreover, in practice *N* can be very large, and the iterative process reaches sufficient accuracy already far earlier. The analysis of these methods is hard, depending on a complicated function of the spectrum of the operator.

The approximating operator that appears in stationary iterative methods can also be incorporated in Krylov subspace methods such as GMRES (alternatively, preconditioned Krylov methods can be considered as accelerations of stationary iterative methods), where they become transformations of the original operator to a presumably better conditioned one. The construction of preconditioners is a large research area.

Probably the first iterative method for solving a linear system appeared in a letter of Gauss to a student of his. He proposed solving a 4-by-4 system of equations by repeatedly solving the component in which the residual was the largest ^{[ citation needed ]}.

The theory of stationary iterative methods was solidly established with the work of D.M. Young starting in the 1950s. The conjugate gradient method was also invented in the 1950s, with independent developments by Cornelius Lanczos, Magnus Hestenes and Eduard Stiefel, but its nature and applicability were misunderstood at the time. Only in the 1970s was it realized that conjugacy based methods work very well for partial differential equations, especially the elliptic type.

In mathematics, an **eigenfunction** of a linear operator *D* defined on some function space is any non-zero function *f* in that space that, when acted upon by *D*, is only multiplied by some scaling factor called an eigenvalue. As an equation, this condition can be written as

In numerical linear algebra, the **Arnoldi iteration** is an eigenvalue algorithm and an important example of an iterative method. Arnoldi finds an approximation to the eigenvalues and eigenvectors of general matrices by constructing an orthonormal basis of the Krylov subspace, which makes it particularly useful when dealing with large sparse matrices.

The **Gauss–Newton algorithm** is used to solve non-linear least squares problems. It is a modification of Newton's method for finding a minimum of a function. Unlike Newton's method, the Gauss–Newton algorithm can only be used to minimize a sum of squared function values, but it has the advantage that second derivatives, which can be challenging to compute, are not required.

In mathematics, the **conjugate gradient method** is an algorithm for the numerical solution of particular systems of linear equations, namely those whose matrix is positive-definite. The conjugate gradient method is often implemented as an iterative algorithm, applicable to sparse systems that are too large to be handled by a direct implementation or other direct methods such as the Cholesky decomposition. Large sparse systems often arise when numerically solving partial differential equations or optimization problems.

The **Rayleigh–Ritz method** is a direct numerical method of approximating eigenvalue, originated in the context of solving physical boundary value problems and named after Lord Rayleigh and Walther Ritz.

In linear algebra, an **eigenvector** or **characteristic vector** of a linear transformation is a nonzero vector that changes at most by a scalar factor when that linear transformation is applied to it. The corresponding **eigenvalue**, often denoted by , is the factor by which the eigenvector is scaled.

In linear algebra, the order-*r***Krylov subspace** generated by an *n*-by-*n* matrix *A* and a vector *b* of dimension *n* is the linear subspace spanned by the images of *b* under the first *r* powers of *A*, that is,

In numerical linear algebra, the **Gauss–Seidel method**, also known as the **Liebmann method** or the **method of successive displacement**, is an iterative method used to solve a system of linear equations. It is named after the German mathematicians Carl Friedrich Gauss and Philipp Ludwig von Seidel, and is similar to the Jacobi method. Though it can be applied to any matrix with non-zero elements on the diagonals, convergence is only guaranteed if the matrix is either strictly diagonally dominant, or symmetric and positive definite. It was only mentioned in a private letter from Gauss to his student Gerling in 1823. A publication was not delivered before 1874 by Seidel.

In numerical linear algebra, the **Jacobi method** is an iterative algorithm for determining the solutions of a strictly diagonally dominant system of linear equations. Each diagonal element is solved for, and an approximate value is plugged in. The process is then iterated until it converges. This algorithm is a stripped-down version of the Jacobi transformation method of matrix diagonalization. The method is named after Carl Gustav Jacob Jacobi.

In numerical linear algebra, the method of **successive over-relaxation** (**SOR**) is a variant of the Gauss–Seidel method for solving a linear system of equations, resulting in faster convergence. A similar method can be used for any slowly converging iterative process.

In mathematics, the **generalized minimal residual method (GMRES)** is an iterative method for the numerical solution of a indefinite nonsymmetric system of linear equations. The method approximates the solution by the vector in a Krylov subspace with minimal residual. The Arnoldi iteration is used to find this vector.

In statistics, **generalized least squares** (**GLS**) is a technique for estimating the unknown parameters in a linear regression model when there is a certain degree of correlation between the residuals in a regression model. In these cases, ordinary least squares and weighted least squares can be statistically inefficient, or even give misleading inferences. GLS was first described by Alexander Aitken in 1936.

In mathematics, **preconditioning** is the application of a transformation, called the **preconditioner**, that conditions a given problem into a form that is more suitable for numerical solving methods. Preconditioning is typically related to reducing a condition number of the problem. The preconditioned problem is then usually solved by an iterative method.

In computational chemistry, a **constraint algorithm** is a method for satisfying the Newtonian motion of a rigid body which consists of mass points. A restraint algorithm is used to ensure that the distance between mass points is maintained. The general steps involved are: (i) choose novel unconstrained coordinates, (ii) introduce explicit constraint forces, (iii) minimize constraint forces implicitly by the technique of Lagrange multipliers or projection methods.

In numerical linear algebra, the **alternating-direction implicit (ADI) method** is an iterative method used to solve Sylvester matrix equations. It is a popular method for solving the large matrix equations that arise in systems theory and control, and can be formulated to construct solutions in a memory-efficient, factored form. It is also used to numerically solve parabolic and elliptic partial differential equations, and is a classic method used for modeling heat conduction and solving the diffusion equation in two or more dimensions. It is an example of an operator splitting method.

The **finite element method** (**FEM**) is a widely used method for numerically solving differential equations arising in engineering and mathematical modeling. Typical problem areas of interest include the traditional fields of structural analysis, heat transfer, fluid flow, mass transport, and electromagnetic potential.

**Linear least squares** (**LLS**) is the least squares approximation of linear functions to data. It is a set of formulations for solving statistical problems involved in linear regression, including variants for ordinary (unweighted), weighted, and generalized (correlated) residuals. Numerical methods for linear least squares include inverting the matrix of the normal equations and orthogonal decomposition methods.

The **conjugate residual method** is an iterative numeric method used for solving systems of linear equations. It's a Krylov subspace method very similar to the much more popular conjugate gradient method, with similar construction and convergence properties.

In the mathematical discipline of numerical linear algebra, a **matrix splitting** is an expression which represents a given matrix as a sum or difference of matrices. Many iterative methods depend upon the direct solution of matrix equations involving matrices more general than tridiagonal matrices. These matrix equations can often be solved directly and efficiently when written as a matrix splitting. The technique was devised by Richard S. Varga in 1960.

In mathematics, **Anderson acceleration**, also called **Anderson mixing**, is a method for the acceleration of the convergence rate of fixed-point iterations. Introduced by Donald G. Anderson, this technique can be used to find the solution to fixed point equations often arising in the field of computational science.

- ↑ Amritkar, Amit; de Sturler, Eric; Świrydowicz, Katarzyna; Tafti, Danesh; Ahuja, Kapil (2015). "Recycling Krylov subspaces for CFD applications and a new hybrid recycling solver".
*Journal of Computational Physics*.**303**: 222. arXiv: 1501.03358 . Bibcode:2015JCoPh.303..222A. doi:10.1016/j.jcp.2015.09.040.

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