This function computes the performance index for a real symmetric eigensystem in band symmetric storage mode.

Required Arguments

EPISB — Performance index.   (Output)

Required Arguments

NEVAL — Number of eigenvalue/eigenvector pairs on which the performance is based.   (Input)

A — Band symmetric matrix of order N.   (Input)

NCODA — Number of codiagonals in A.   (Input)

EVAL — Vector of length NEVAL containing eigenvalues of A.   (Input)

EVEC — N by NEVAL array containing eigenvectors of A.   (Input)
The eigenvector corresponding to the eigenvalue EVAL(J) must be in the J-th column of EVEC.

Optional Arguments

N — Order of the matrix A.   (Input)
Default: N = size (A,2).

LDA — Leading dimension of A exactly as specified in the dimension statement in the calling program.   (Input)
Default: LDA = size (A,1).

LDEVEC — Leading dimension of EVEC exactly as specified in the dimension statement in the calling program.   (Input)
Default: LDEVEC = size (EVEC,1).

FORTRAN 90 Interface

Generic:          EPISB (NEVAL, A, NCODA, EVAL, EVEC [,…])

Specific:         The specific interface names are S_EPISB and D_EPISB.

FORTRAN 77 Interface

Single:            EPISB (N, NEVAL, A, LDA, NCODA, EVAL, EVEC, LDEVEC)

Double:          The double precision function name is DEPISB.

Description

Let M = NEVAL, l = EVAL, xj = EVEC(*,J), the j-th column of EVEC. Also, let ε be the machine precision, given by AMACH(4), see the Reference chapter of the manual. The performance index, τ, is defined to be

While the exact value of τ is highly machine dependent, the performance of EVCSF is considered excellent if τ < 1, good if 1 ≤ τ ≤ 100, and poor if τ > 100. The performance index was first developed by the EISPACK project at Argonne National Laboratory; see Smith et al. (1976, pages 124− 125).

Comments

1.         Workspace may be explicitly provided, if desired, by use of E2ISB/DE2ISB. The reference is:

E2ISB (N, NEVAL, A, LDA, NCODA, EVAL, EVEC, LDEVEC, WK)

The additional argument is:

WK — Work array of length N.

2.         Informational errors

Type   Code

3           1                  Performance index is greater than 100.

3           2                  An eigenvector is zero.

3           3                  The matrix is zero.

Example

For an example of EPISB, see IMSL routine EVCSB.

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