Modify the Problem Dependent Variables

  To set up the proper storage and to arrange for effective use of the IRAM various parameters will have to be set. These variables include:

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• [n] The dimension of the problem.
• [nev] The number of eigenvalues needed.
• [ncv] The length of the Arnoldi factorization. This represents the maximum number of Arnoldi vectors used.
• [which] The eigenvalues of interest.
• [info] Set to 0 for a randomly generated starting vector. If the user decides to use another starting vector, this value should be set to 1, and the starting vector should be provided in the array resid.
• [sigmar] The real part of the shift used if a spectral transformation is employed.
• [sigmai] The imaginary part of the shift used if a spectral transformation is employed.

The variable nev may be set to be a value larger than the number of eigenvalues desired to avoid splitting an eigenvalue cluster. The only restriction is that nev must be less than ncv. The recommended choice of ncv is to set ncv = 2 nev. The user is encouraged to experiment with both nev and ncv. The possible choices for the input variable which are listed in Table A.6.

 

which EIGENVALUES

'LM' Largest magnitude

'SM' Smallest magnitude

'LR' Largest real parts

'SR' Smallest real parts

'LI' Largest imaginary parts

'SI' Smallest imaginary parts

When using a spectral transformation, the selection of which = 'SM' should be avoided.

Once the above variables are modified, the storage declarations

      integer          maxn, maxnev, maxncv, ldv
      parameter       (maxn=256, maxnev=10, maxncv=25, ldv=maxn )

should be adjusted so that the conditions

n maxn,

nev maxnev,

ncv maxncv,

nev+2 ncv

are satisfied. The last condition on nev is needed to assure that complex conjugate pairs of eigenvalues are kept together.