NAG Library Function Document

nag_real_symm_sparse_eigensystem_sol (f12fcc)

1  Purpose

2  Specification

3  Description

4  References

5  Arguments

1:    $\mathbf{nconv}$ – Integer *Output

On exit: the number of converged eigenvalues as found by nag_real_symm_sparse_eigensystem_iter (f12fbc).

2:    $\mathbf{d}\left[\mathit{dim}\right]$ – doubleOutput

Note: the dimension, dim, of the array d must be at least $\mathbf{ncv}$ (see nag_real_symm_sparse_eigensystem_init (f12fac)).

On exit: the first nconv locations of the array d contain the converged approximate eigenvalues.

3:    $\mathbf{z}\left[\mathbf{n}\times \left(\mathbf{nev}+1\right)\right]$ – doubleOutput

On exit: if the default option $\mathbf{Vectors}=\mathrm{RITZ}$ (see nag_real_symm_sparse_eigensystem_option (f12fdc)) has been selected then z contains the final set of eigenvectors corresponding to the eigenvalues held in d. The real eigenvector associated with an eigenvalue is stored in the corresponding array section of z.

4:    $\mathbf{sigma}$ – doubleInput

On entry: if one of the $\mathbf{Shifted\; Inverse}$ (see nag_real_symm_sparse_eigensystem_option (f12fdc)) modes has been selected then sigma contains the real shift used; otherwise sigma is not referenced.

5:    $\mathbf{resid}\left[\mathit{dim}\right]$ – const doubleInput

Note: the dimension, dim, of the array resid must be at least $\mathbf{n}$ (see nag_real_symm_sparse_eigensystem_init (f12fac)).

On entry: must not be modified following a call to nag_real_symm_sparse_eigensystem_iter (f12fbc) since it contains data required by nag_real_symm_sparse_eigensystem_sol (f12fcc).

6:    $\mathbf{v}\left[\mathbf{n}\times \mathbf{ncv}\right]$ – doubleInput/Output

The $\mathit{i}$th element of the $\mathit{j}$th basis vector is stored in location $\mathbf{v}\left[\mathbf{n}\times \left(\mathit{j}-1\right)+\mathit{i}-1\right]$, for $\mathit{i}=1,2,\dots ,\mathbf{n}$ and $\mathit{j}=1,2,\dots ,\mathbf{ncv}$.

On entry: the ncv sections of v, of length $n$, contain the Lanczos basis vectors for $\mathrm{OP}$ as constructed by nag_real_symm_sparse_eigensystem_iter (f12fbc).

On exit: if the option $\mathbf{Vectors}=\mathrm{SCHUR}$ has been set, or the option $\mathbf{Vectors}=\mathrm{RITZ}$ has been set and a separate array z has been passed (i.e., z does not equal v), then the first nconv sections of v, of length $n$, will contain approximate Schur vectors that span the desired invariant subspace.

7:    $\mathbf{comm}\left[\mathit{dim}\right]$ – doubleCommunication Array

Note: the dimension, dim, of the array comm must be at least $\max \left(1,\mathbf{lcomm}\right)$ (see nag_real_symm_sparse_eigensystem_init (f12fac)).

On initial entry: must remain unchanged from the prior call to nag_real_symm_sparse_eigensystem_init (f12fac).

On exit: contains data on the current state of the solution.

8:    $\mathbf{icomm}\left[\mathit{dim}\right]$ – IntegerCommunication Array

Note: the dimension, dim, of the array icomm must be at least $\max \left(1,\mathbf{licomm}\right)$ (see nag_real_symm_sparse_eigensystem_init (f12fac)).

On initial entry: must remain unchanged from the prior call to nag_real_symm_sparse_eigensystem_init (f12fac).

On exit: contains data on the current state of the solution.

9:    $\mathbf{fail}$ – NagError *Input/Output

The NAG error argument (see Section 3.6 in the Essential Introduction).

6  Error Indicators and Warnings

NE_ALLOC_FAIL

Dynamic memory allocation failed.
See Section 3.2.1.2 in the Essential Introduction for further information.

NE_BAD_PARAM

On entry, argument $〈\mathit{\text{value}}〉$ had an illegal value.

NE_INTERNAL_ERROR

An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.

An unexpected error has been triggered by this function. Please contact NAG.
See Section 3.6.6 in the Essential Introduction for further information.

NE_INVALID_OPTION

On entry, $\mathbf{Vectors}=\mathrm{SELECT}$, but this is not yet implemented.

NE_MAX_ITER

During calculation of a tridiagonal form, there was a failure to compute $〈\mathit{\text{value}}〉$ eigenvalues in a total of $〈\mathit{\text{value}}〉$ iterations.

NE_NO_LICENCE

Your licence key may have expired or may not have been installed correctly.
See Section 3.6.5 in the Essential Introduction for further information.

NE_RITZ_COUNT

Got a different count of the number of converged Ritz values than the value passed to it through the argument icomm: number counted $=〈\mathit{\text{value}}〉$, number expected $=〈\mathit{\text{value}}〉$. This usually indicates that a communication array has been altered or has become corrupted between calls to nag_real_symm_sparse_eigensystem_iter (f12fbc) and nag_real_symm_sparse_eigensystem_sol (f12fcc).

NE_ZERO_EIGS_FOUND

The number of eigenvalues found to sufficient accuracy, as communicated through the argument icomm, is zero. You should experiment with different values of nev and ncv, or select a different computational mode or increase the maximum number of iterations prior to calling nag_real_symm_sparse_eigensystem_iter (f12fbc).

7  Accuracy

8  Parallelism and Performance

9  Further Comments

10  Example

10.1  Program Text

Program Text (f12fcce.c)

10.2  Program Data

Program Data (f12fcce.d)

10.3  Program Results

Program Results (f12fcce.r)