NAG FL Interface
f06cbf (zrotgs)

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1 Purpose

f06cbf generates a complex Givens plane rotation having real sine.

2 Specification

Fortran Interface
Subroutine f06cbf ( a, b, c, s)
Real (Kind=nag_wp), Intent (Out) :: s
Complex (Kind=nag_wp), Intent (Inout) :: a, b
Complex (Kind=nag_wp), Intent (Out) :: c
C Header Interface
#include <nag.h>
void  f06cbf_ (Complex *a, Complex *b, Complex *c, double *s)
The routine may be called by the names f06cbf or nagf_blas_zrotgs.

3 Description

f06cbf generates a complex Givens plane rotation with parameters c (complex) and s (real), such that, given complex a and b:
( c¯ s -s c ) ( a b )=( d 0 ) ,  
If b is real, then d is also real. 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 f06cdf.
If |b|<ε|a| and Im(a)=0, where ε is the machine precision, the routine sets c=1 and s=|t|.
If |b|<ε|a| and Im(a)0, the routine sets
c=sign(Re(a))a |a|   and  s=|t|  
Note that t is always set to b/a, unless overflow would occur, in which case the routine returns the value of the complex expression
( flmax × sign(Re(b)/a) , flmax × sign(Im(b)/a) ) ;  
flmax is the real value given by 1/(x02amf).

4 References

None.

5 Arguments

1: a Complex (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 Complex (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 Complex (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

Background information to multithreading can be found in the Multithreading documentation.
f06cbf is not threaded in any implementation.

9 Further Comments

None.

10 Example

None.