NAG Library Routine Document

f06aaf  (drotg)

 Contents

    1  Purpose
    7  Accuracy
    10  Example

1
Purpose

f06aaf (drotg) generates a real Givens plane rotation.

2
Specification

Fortran Interface
Subroutine f06aaf ( a, b, c, s)
Real (Kind=nag_wp), Intent (Inout):: a, b
Real (Kind=nag_wp), Intent (Out):: c, s
C Header Interface
#include nagmk26.h
void  f06aaf_ ( double *a, double *b, double *c, double *s)
The routine may be called by its BLAS name drotg.

3
Description

f06aaf (drotg) generates a real Givens plane rotation with parameters c and s, such that, given real a and b:
c s -s c a b = d 0 .  
The routine computes c, s and d as follows:
d = σa2+b2 ;  
c = a/d, if ​ d 0, 1, if ​ d= 0, s = b/d, if ​ d 0, 0, if ​ d= 0,  
where   σ = signa, if ​a>b, signb, if ​ab.  
The routine also computes the value z defined as
z= s, if ​s<c​ or ​c=0, 1/c, if ​0<cs.  
This enables c and s to be reconstructed from the single value z as
c= 1-z2, if ​z1, 1/z, if ​z>1, s= z, if ​z1, 1-c2, if ​z>1.  
To apply the plane rotation to a pair of real vectors, call f06epf (drot); to apply it to a pair of complex vectors, call f06kpf (zdrot).

4
References

None.

5
Arguments

1:     a – Real (Kind=nag_wp)Input/Output
On entry: the value a, the first element of the vector which determines the rotation.
On exit: the value d.
2:     b – Real (Kind=nag_wp)Input/Output
On entry: the value b, the second element of the vector which determines the rotation.
On exit: the value z, from which c and s can be reconstructed.
3:     c – Real (Kind=nag_wp)Output
On exit: the value c, the cosine of the rotation.
4:     s – Real (Kind=nag_wp)Output
On exit: the value s, the sine of the rotation.

6
Error Indicators and Warnings

None.

7
Accuracy

Not applicable.

8
Parallelism and Performance

f06aaf (drotg) is not threaded in any implementation.

9
Further Comments

None.

10
Example

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