Editing Iterative method (section)

====Convergence of Krylov subspace methods====
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 an operator|spectrum]] of the operator.