In this section, the simple code dssimp is discussed. All of the other example drivers are similar in nature. This particular example program illustrates the simplest computational mode of using ARPACK in considerable detail. dssimp shows how to use ARPACK to find a few eigenvalues and corresponding eigenvectors for the standard eigenvalue problem:
where is an n by n real symmetric matrix. The main points illustrated are:This dssimp program is a driver for the subroutine dsaupd and it is set up to solve the following problem:
The only thing that must be supplied in order to use this routine on your problem is to change the array dimensions and to supply a means to compute the matrix-vector product
on request from dsaupd. The selection of which eigenvalues to compute may be altered by changing the parameter which.Once usage of dsaupd in the simplest mode is understood, you may wish to explore the other available options such as solving generalized eigenvalue problems using a shift-invert computational mode. Some of these additional modes are described in the latter sections of this chapter and also in the file ex-sym.doc in DOCUMENTS directory.