# Simple-iteration method

A method for approximately solving a system of linear algebraic equations $ Ax = b $
that can be transformed to the form $ x = Bx + c $
and whose solution is looked for as the limit of a sequence $ x ^ {k+1} = B x ^ {k} + c $,
$ k = 0 , 1 \dots $
where $ x ^ {0} $
is an initial approximation. In order that the simple-iteration method converges for any initial approximation $ x ^ {0} $
it is necessary and sufficient that all eigenvalues of $ B $
are less than one in modulus; it is sufficient that some norm of $ B $
is less than one. If in some norm, compatible with the norm of a vector $ x $,
$ B $
satisfies $ \| B \| \leq \rho < 1 $,
then the simple-iteration method converges at the rate of a geometric series and the estimate

$$ \| x ^ {m} - x \| \leq \rho ^ {m} \| x ^ {0} - x \| $$

holds for its error.

In the case of a cubic, octahedral or spherical vector norm, the condition $ \| B \| \leq \rho $ is fulfilled if

1) $ \sum _ {j=1} ^ {n} | b _ {ij} | \leq \rho $, $ i = 1 \dots n $;

2) $ \sum _ {i=1} ^ {n} | b _ {ij} | \leq \rho $, $ j = 1 \dots n $;

3) $ \sum _ {i , j = 1 } ^ {n} b _ {ij} ^ {2} \leq \rho ^ {2} $.

The simplest version of the method corresponds to the case when one takes $ I - A $, where $ I $ is the identity matrix, for $ B $. If all diagonal entries of $ A $ are non-zero, then, choosing $ b = D ^ {-1} ( D - A ) $ and $ c = D ^ {-1} b $, where $ D $ is the diagonal matrix with as diagonal entries those of $ A $, one obtains the Jacobi method or the method of simultaneous displacement.

A particular case of the simple-iteration method is the method with $ B = I - \tau A $ and $ c = \tau b $, where $ \tau $ is an iteration parameter, chosen from the condition that the norm of $ I - \tau A $ is minimal with respect to $ \tau $. If $ \gamma _ {1} $ and $ \gamma _ {2} $ are the minimal and maximal eigenvalues of a symmetric positive-definite matrix $ A $ and $ \tau = 2 / ( \gamma _ {1} + \gamma _ {2} ) $, then one has for the matrix $ B $ in the spherical norm the estimate $ \| B \| \leq \rho $, with $ \rho = ( \gamma _ {2} - \gamma _ {1} ) / ( \gamma _ {2} + \gamma _ {1} ) < 1 $.

For a system of non-linear algebraic equations

$$ \phi _ {i} ( x) = 0 ,\ 1 \leq i \leq n ,\ x = ( x _ {1} \dots x _ {n} ) , $$

the simple-iteration method has the form

$$ x _ {i} ^ {k+1} = x _ {i} ^ {k} - \tau \phi _ {i} ( x ^ {k} ) ,\ 1 \leq i \leq n ,\ k \geq 0 . $$

The problem of choosing the iteration parameter $ \tau $ is solved in dependence on the differentiability properties of the $ \phi _ {i} $. Often it is subjected to the requirement that the method converges locally in a neighbourhood of a solution.

#### References

[1] | D.K. Faddeev, V.N. Faddeeva, "Computational methods of linear algebra" , Freeman (1963) (Translated from Russian) |

[2] | I.S. Berezin, N.P. Zhidkov, "Computing methods" , Pergamon (1973) (Translated from Russian) |

[3] | J.M. Ortega, W.C. Rheinboldt, "Iterative solution of non-linear equations in several variables" , Acad. Press (1970) |

[4] | A.A. Samarskii, E.S. Nikolaev, "Numerical methods for grid equations" , 1–2 , Birkhäuser (1989) (Translated from Russian) |

**How to Cite This Entry:**

Simple-iteration method.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Simple-iteration_method&oldid=54970