# NAG CL Interfacef12acc (real_​proc)

Note: this function uses optional parameters to define choices in the problem specification. If you wish to use default settings for all of the optional parameters, then the option setting function f12adc need not be called. If, however, you wish to reset some or all of the settings please refer to Section 11 in f12adc for a detailed description of the specification of the optional parameters.

## 1Purpose

f12acc is a post-processing function that must be called following a final exit from f12abc. These are part of a suite of functions for the solution of real sparse eigensystems. The suite also includes f12aac, f12adc and f12aec.

## 2Specification

 #include
 void f12acc (Integer *nconv, double dr[], double di[], double z[], double sigmar, double sigmai, const double resid[], double v[], double comm[], Integer icomm[], NagError *fail)
The function may be called by the names: f12acc, nag_sparseig_real_proc or nag_real_sparse_eigensystem_sol.

## 3Description

The suite of functions is designed to calculate some of the eigenvalues, $\lambda$, (and optionally the corresponding eigenvectors, $x$) of a standard eigenvalue problem $Ax=\lambda x$, or of a generalized eigenvalue problem $Ax=\lambda Bx$ of order $n$, where $n$ is large and the coefficient matrices $A$ and $B$ are sparse, real and nonsymmetric. The suite can also be used to find selected eigenvalues/eigenvectors of smaller scale dense, real and nonsymmetric problems.
Following a call to f12abc, f12acc returns the converged approximations to eigenvalues and (optionally) the corresponding approximate eigenvectors and/or an orthonormal basis for the associated approximate invariant subspace. The eigenvalues (and eigenvectors) are selected from those of a standard or generalized eigenvalue problem defined by real nonsymmetric matrices. There is negligible additional cost to obtain eigenvectors; an orthonormal basis is always computed, but there is an additional storage cost if both are requested.
f12acc is based on the function dneupd from the ARPACK package, which uses the Implicitly Restarted Arnoldi iteration method. The method is described in Lehoucq and Sorensen (1996) and Lehoucq (2001) while its use within the ARPACK software is described in great detail in Lehoucq et al. (1998). An evaluation of software for computing eigenvalues of sparse nonsymmetric matrices is provided in Lehoucq and Scott (1996). This suite of functions offers the same functionality as the ARPACK software for real nonsymmetric problems, but the interface design is quite different in order to make the option setting clearer and to simplify some of the interfaces.
f12acc, is a post-processing function that must be called following a successful final exit from f12abc. f12acc uses data returned from f12abc and options, set either by default or explicitly by calling f12adc, to return the converged approximations to selected eigenvalues and (optionally):
• the corresponding approximate eigenvectors;
• an orthonormal basis for the associated approximate invariant subspace;
• both.

## 4References

Lehoucq R B (2001) Implicitly restarted Arnoldi methods and subspace iteration SIAM Journal on Matrix Analysis and Applications 23 551–562
Lehoucq R B and Scott J A (1996) An evaluation of software for computing eigenvalues of sparse nonsymmetric matrices Preprint MCS-P547-1195 Argonne National Laboratory
Lehoucq R B and Sorensen D C (1996) Deflation techniques for an implicitly restarted Arnoldi iteration SIAM Journal on Matrix Analysis and Applications 17 789–821
Lehoucq R B, Sorensen D C and Yang C (1998) ARPACK Users' Guide: Solution of Large-scale Eigenvalue Problems with Implicitly Restarted Arnoldi Methods SIAM, Philidelphia

## 5Arguments

1: $\mathbf{nconv}$Integer * Output
On exit: the number of converged eigenvalues as found by f12abc.
2: $\mathbf{dr}\left[\mathit{dim}\right]$double Output
Note: the dimension, dim, of the array dr must be at least ${\mathbf{nev}}+1$ (see f12aac).
On exit: the first nconv locations of the array dr contain the real parts of the converged approximate eigenvalues.
3: $\mathbf{di}\left[\mathit{dim}\right]$double Output
Note: the dimension, dim, of the array di must be at least ${\mathbf{nev}}+1$ (see f12aac).
On exit: the first nconv locations of the array di contain the imaginary parts of the converged approximate eigenvalues.
4: $\mathbf{z}\left[{\mathbf{n}}×\left({\mathbf{nev}}+1\right)\right]$double Output
On exit: if the default option ${\mathbf{Vectors}}=\mathrm{RITZ}$ (see f12adc) has been selected then z contains the final set of eigenvectors corresponding to the eigenvalues held in dr and di. The complex eigenvector associated with the eigenvalue with positive imaginary part is stored in two consecutive array segments. The first segment holds the real part of the eigenvector and the second holds the imaginary part. The eigenvector associated with the eigenvalue with negative imaginary part is simply the complex conjugate of the eigenvector associated with the positive imaginary part.
For example, the first eigenvector has real parts stored in locations ${\mathbf{z}}\left[\mathit{i}-1\right]$, for $\mathit{i}=1,2,\dots ,{\mathbf{n}}$ and imaginary parts stored in ${\mathbf{z}}\left[\mathit{i}-1\right]$, for $\mathit{i}={\mathbf{n}}+1,2{\mathbf{n}}$.
5: $\mathbf{sigmar}$double Input
On entry: if one of the ${\mathbf{Shifted Inverse Real}}$ modes have been selected then sigmar contains the real part of the shift used; otherwise sigmar is not referenced.
6: $\mathbf{sigmai}$double Input
On entry: if one of the ${\mathbf{Shifted Inverse Real}}$ modes have been selected then sigmai contains the imaginary part of the shift used; otherwise sigmai is not referenced.
7: $\mathbf{resid}\left[\mathit{dim}\right]$const double Input
Note: the dimension, dim, of the array resid must be at least ${\mathbf{n}}$ (see f12aac).
On entry: must not be modified following a call to f12abc since it contains data required by f12acc.
8: $\mathbf{v}\left[{\mathbf{n}}×{\mathbf{ncv}}\right]$double Input/Output
The $\mathit{i}$th element of the $\mathit{j}$th basis vector is stored in location ${\mathbf{v}}\left[{\mathbf{n}}×\left(\mathit{j}-1\right)+\mathit{i}-1\right]$, 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 Arnoldi basis vectors for $\mathrm{OP}$ as constructed by f12abc.
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.
9: $\mathbf{comm}\left[\mathit{dim}\right]$double Communication Array
Note: the actual argument supplied must be the array comm supplied to the initialization routine f12aac.
On initial entry: must remain unchanged from the prior call to f12abc.
On exit: contains data on the current state of the solution.
10: $\mathbf{icomm}\left[\mathit{dim}\right]$Integer Communication Array
Note: the actual argument supplied must be the array icomm supplied to the initialization routine f12aac.
On initial entry: must remain unchanged from the prior call to f12abc.
On exit: contains data on the current state of the solution.
11: $\mathbf{fail}$NagError * Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

## 6Error Indicators and Warnings

NE_ALLOC_FAIL
Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
On entry, argument $〈\mathit{\text{value}}〉$ had an illegal value.
NE_INITIALIZATION
Either the solver function has not been called prior to the call of this function or a communication array has become corrupted.
NE_INTERNAL_EIGVEC_FAIL
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.
See Section 7.5 in the Introduction to the NAG Library CL Interface for further information.
NE_INVALID_OPTION
On entry, ${\mathbf{Vectors}}=\mathrm{SELECT}$, but this is not yet implemented.
NE_NO_LICENCE
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library CL Interface 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 f12abc and f12acc.
NE_SCHUR_EIG_FAIL
During calculation of a real Schur form, there was a failure to compute $〈\mathit{\text{value}}〉$ eigenvalues in a total of $〈\mathit{\text{value}}〉$ iterations.
NE_SCHUR_REORDER
The computed Schur form could not be reordered by an internal call. This function returned with ${\mathbf{fail}}\mathbf{.}\mathbf{code}=〈\mathit{\text{value}}〉$. Please contact NAG.
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 f12abc.

## 7Accuracy

The relative accuracy of a Ritz value, $\lambda$, is considered acceptable if its Ritz estimate $\le {\mathbf{Tolerance}}×\left|\lambda \right|$. The default ${\mathbf{Tolerance}}$ used is the machine precision given by X02AJC.

## 8Parallelism and Performance

f12acc makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this function. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

None.

## 10Example

This example solves $Ax=\lambda Bx$ in regular-invert mode, where $A$ and $B$ are obtained from the standard central difference discretization of the one-dimensional convection-diffusion operator $\frac{{d}^{2}u}{d{x}^{2}}+\rho \frac{du}{dx}$ on $\left[0,1\right]$, with zero Dirichlet boundary conditions.

### 10.1Program Text

Program Text (f12acce.c)

### 10.2Program Data

Program Data (f12acce.d)

### 10.3Program Results

Program Results (f12acce.r)