NAG Library Routine Document

f06baf  (drotgc)

 Contents

    1  Purpose
    7  Accuracy
    10  Example

1
Purpose

f06baf generates a real Givens plane rotation and the tangent of that rotation.

2
Specification

Fortran Interface
Subroutine f06baf ( 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  f06baf_ ( double *a, double *b, double *c, double *s)

3
Description

f06baf generates a real Givens plane rotation with parameters c (0) and s, such that, given real a and b:
c s -s c a b = d 0 .  
On exit, b is overwritten by t, the tangent of the rotation; c and s can be reconstructed from the single stored value t, by a subsequent call to f06bcf.
If b<εa, where ε is the machine precision, the routine sets c=1 and s=0; if a<εb, the routine sets c=0 and s=signb/a.
Note that t is always set to b/a, unless this would overflow, in which case the value flmax×signb/a is returned, where flmax is the value given by 1/x02amf.
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 t, the tangent of the rotation.
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

f06baf is not threaded in any implementation.

9
Further Comments

None.

10
Example

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