Editing Iterative method (section)

====Definition====
An ''iterative method'' is defined by
:<math>
  \mathbf{x}^{k+1} := \Psi (  \mathbf{x}^k ), \quad k \geq 0
</math>
and for a given linear system <math> A\mathbf x= \mathbf b </math> with exact solution <math> \mathbf{x}^* </math> the ''error'' by
:<math>
  \mathbf{e}^k := \mathbf{x}^k - \mathbf{x}^*, \quad k \geq 0.
</math>
An iterative method is called ''linear'' if there exists a matrix <math> C \in \R^{n\times n} </math> such that
:<math>
  \mathbf{e}^{k+1} = C  \mathbf{e}^k   \quad \forall k \geq 0
</math>
and this matrix is called the ''iteration matrix''.
An iterative method with a given iteration matrix <math> C </math> is called ''convergent'' if the following holds
:<math>
  \lim_{k\rightarrow \infty} C^k=0.
</math>

An important theorem states that for a given iterative method and its iteration matrix <math> C </math> it is convergent if and only if its [[spectral radius]] <math> \rho(C) </math>  is smaller than unity, that is,
:<math>
  \rho(C) < 1.
</math>

The basic iterative methods work by [[Matrix splitting|splitting]] the matrix <math> A </math> into
:<math>
  A = M - N
</math>
and here the matrix <math> M </math> should be easily [[Invertible matrix|invertible]].
The iterative methods are now defined as
:<math>
  M \mathbf{x}^{k+1} = N \mathbf{x}^k + b, \quad k \geq 0,
</math>
or, equivalently,
:<math>
  \mathbf{x}^{k+1} = \mathbf{x}^k + M^{-1} (b - A \mathbf{x}^k), \quad k \geq 0.
</math>
From this follows that the iteration matrix is given by
:<math>
  C = I - M^{-1}A = M^{-1}N.
</math>