F06BPF (PDF version)
F06 Chapter Contents
F06 Chapter Introduction
NAG Library Manual

NAG Library Routine Document

F06BPF

Note:  before using this routine, please read the Users' Note for your implementation to check the interpretation of bold italicised terms and other implementation-dependent details.

▸▿ Contents

    1  Purpose
    2  Specification
    3  Description
    4  References
    5  Parameters
    6  Error Indicators and Warnings
    7  Accuracy
    8  Parallelism and Performance
    9  Further Comments
    10  Example

1  Purpose

F06BPF returns an eigenvalue of a 2 by 2 real symmetric matrix.

2  Specification

FUNCTION F06BPF ( A, B, C)
REAL (KIND=nag_wp) F06BPF
REAL (KIND=nag_wp)  A, B, C

3  Description

F06BPF returns an eigenvalue of the 2 by 2 real symmetric matrix
a b b c ,  
via the function name. The result is intended for use as a shift in symmetric eigenvalue routines.
The eigenvalue is computed as
c - b f + sign⁡f × 1+f2 ,  
where f = a-c 2b .
This is the eigenvalue nearer to c if a≠c, and is equal to c-b if a=c.

4  References

None.

5  Parameters

1:     A – REAL (KIND=nag_wp)Input
On entry: the value a, the 1,1 element of the input matrix.
2:     B – REAL (KIND=nag_wp)Input
On entry: the value b, the 1,2 or 2,1 element of the input matrix.
3:     C – REAL (KIND=nag_wp)Input
On entry: the value c, the 2,2 element of the input matrix.

6  Error Indicators and Warnings

None.

7  Accuracy

Not applicable.

8  Parallelism and Performance

Not applicable.

9  Further Comments

None.

10  Example

None.
F06BPF (PDF version)
F06 Chapter Contents
F06 Chapter Introduction
NAG Library Manual
© The Numerical Algorithms Group Ltd, Oxford, UK. 2015