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 . This means you can get a vector in the middle of the spectrum without solving the matrix.
, with and the Identity matrix.
In contrast to the Conjugate gradient method, here the gradient calculates by twice multiplying matrix
Literature
https://web.archive.org/web/20070806144253/http://www.sst.nrel.gov/topics/nano/escan.html