Folded spectrum method

In mathematics, the folded spectrum method (FSM) is an iterative method for solving large eigenvalue problems.
Here you always find a vector with an eigenvalue close to a search-value formula_1. This means you can get a vector formula_2 in the middle of the spectrum without solving the matrix.
formula_3, with formula_4 and formula_5 the Identity matrix.
In contrast to the Conjugate gradient method, here the gradient calculates by twice multiplying matrix formula_6