NAG Library Routine Document

f06bpf  (deig2)

 Contents

    1  Purpose
    7  Accuracy
    10  Example

1
Purpose

f06bpf returns an eigenvalue of a 2 by 2 real symmetric matrix.

2
Specification

Fortran Interface
Function f06bpf ( a, b, c)
Real (Kind=nag_wp):: f06bpf
Real (Kind=nag_wp), Intent (In):: a, b, c
C Header Interface
#include nagmk26.h
double  f06bpf_ ( const double *a, const double *b, const double *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 + signf × 1+f2 ,  
where f = a-c 2b .
This is the eigenvalue nearer to c if ac, and is equal to c-b if a=c.

4
References

None.

5
Arguments

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

f06bpf is not threaded in any implementation.

9
Further Comments

None.

10
Example

None.
© The Numerical Algorithms Group Ltd, Oxford, UK. 2017