# NAG FL Interfacef12jaf (feast_​init)

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## 1Purpose

f12jaf initializes a data structure for the NAG FEAST suite of routines consisting of f12jaf, f12jbf, f12jef, f12jff, f12jgf, f12jjf, f12jkf, f12jrf, f12jsf, f12jtf, f12juf, f12jvf and f12jzf. It is used to find some of the eigenvalues, and the corresponding eigenvectors, of a standard, generalized or polynomial eigenvalue problem.
The suite of routines is suitable for the solution of large, sparse eigenproblems where only those eigenvalues within a selected region of the complex plane are required.

## 2Specification

Fortran Interface
 Subroutine f12jaf (
 Integer, Intent (Inout) :: ifail Type (c_ptr), Intent (Out) :: handle
#include <nag.h>
 void f12jaf_ (void **handle, Integer *ifail)
The routine may be called by the names f12jaf or nagf_sparseig_feast_init.

## 3Description

The NAG FEAST suite of routines is designed to calculate some of the eigenvalues, $\lambda$, and the corresponding eigenvectors, $x$, of a standard eigenvalue problem $Ax=\lambda x$, a generalized eigenvalue problem $Ax=\lambda Bx$, where $A$ and $B$ are large and sparse, or a polynomial eigenvalue problem ${\sum }_{i}{\lambda }^{i}{A}_{i}x=0$. It can also be used to find eigenvalues/eigenvectors of smaller scale dense problems. The NAG FEAST suite is based on the FEAST library, using contour integration to find the eigenvalues within a contour in the complex plane.
f12jaf is a setup routine which must be called before the option setting routine f12jbf, before the contour definition routines f12jef, f12jff or f12jgf, and before the reverse communication solvers f12jjf, f12jkf, f12jrf, f12jsf, f12jtf, f12juf or f12jvf.
This setup routine initializes the handle to a data structure used internally by the NAG FEAST suite and sets (to their default values) all options that can be set by you via the option setting routine f12jbf. For details of the options available and how to set them see Section 11.1 in f12jbf. When the handle is no longer needed, f12jzf must be called to destroy it and deallocate all the allocated memory and data within.

## 4References

Polizzi E (2009) Density-Matrix-Based Algorithms for Solving Eigenvalue Problems Phys. Rev. B. 79 115112

## 5Arguments

1: $\mathbf{handle}$Type (c_ptr) Output
Note: handle does not need to be set on input.
On exit: holds a handle to the internal data structure used by the NAG FEAST suite.
2: $\mathbf{ifail}$Integer Input/Output
On entry: ifail must be set to $0$, $-1$ or $1$ to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of $0$ causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of $-1$ means that an error message is printed while a value of $1$ means that it is not.
If halting is not appropriate, the value $-1$ or $1$ is recommended. If message printing is undesirable, then the value $1$ is recommended. Otherwise, the value $0$ is recommended. When the value $-\mathbf{1}$ or $\mathbf{1}$ is used it is essential to test the value of ifail on exit.
On exit: ${\mathbf{ifail}}={\mathbf{0}}$ unless the routine detects an error or a warning has been flagged (see Section 6).

## 6Error Indicators and Warnings

If on entry ${\mathbf{ifail}}=0$ or $-1$, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
${\mathbf{ifail}}=-99$
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-399$
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

Not applicable.