.TH "BLAS/SRC/drotg.f90" 3 "Version 3.12.0" "LAPACK" \" -*- nroff -*-
.ad l
.nh
.SH NAME
BLAS/SRC/drotg.f90
.SH SYNOPSIS
.br
.PP
.SS "Functions/Subroutines"

.in +1c
.ti -1c
.RI "subroutine \fBdrotg\fP (a, b, c, s)"
.br
.RI "\fBDROTG\fP "
.in -1c
.SH "Function/Subroutine Documentation"
.PP 
.SS "subroutine drotg (real(wp) a, real(wp) b, real(wp) c, real(wp) s)"

.PP
\fBDROTG\fP 
.PP
\fBPurpose:\fP
.RS 4

.PP
.nf
 DROTG constructs a plane rotation
    [  c  s ] [ a ] = [ r ]
    [ -s  c ] [ b ]   [ 0 ]
 satisfying c**2 + s**2 = 1\&.

 The computation uses the formulas
    sigma = sgn(a)    if |a| >  |b|
          = sgn(b)    if |b| >= |a|
    r = sigma*sqrt( a**2 + b**2 )
    c = 1; s = 0      if r = 0
    c = a/r; s = b/r  if r != 0
 The subroutine also computes
    z = s    if |a| > |b|,
      = 1/c  if |b| >= |a| and c != 0
      = 1    if c = 0
 This allows c and s to be reconstructed from z as follows:
    If z = 1, set c = 0, s = 1\&.
    If |z| < 1, set c = sqrt(1 - z**2) and s = z\&.
    If |z| > 1, set c = 1/z and s = sqrt( 1 - c**2)\&.
.fi
.PP
.RE
.PP
\fBSee also\fP
.RS 4
\fBlartg:          generate plane rotation, more accurate than BLAS rot\fP, 
.PP
\fBlartgp:         generate plane rotation, more accurate than BLAS rot\fP 
.RE
.PP
\fBParameters\fP
.RS 4
\fIA\fP 
.PP
.nf
          A is DOUBLE PRECISION
          On entry, the scalar a\&.
          On exit, the scalar r\&.
.fi
.PP
.br
\fIB\fP 
.PP
.nf
          B is DOUBLE PRECISION
          On entry, the scalar b\&.
          On exit, the scalar z\&.
.fi
.PP
.br
\fIC\fP 
.PP
.nf
          C is DOUBLE PRECISION
          The scalar c\&.
.fi
.PP
.br
\fIS\fP 
.PP
.nf
          S is DOUBLE PRECISION
          The scalar s\&.
.fi
.PP
 
.RE
.PP
\fBAuthor\fP
.RS 4
Edward Anderson, Lockheed Martin 
.RE
.PP
\fBContributors:\fP
.RS 4
Weslley Pereira, University of Colorado Denver, USA 
.RE
.PP
\fBFurther Details:\fP
.RS 4

.PP
.nf
  Anderson E\&. (2017)
  Algorithm 978: Safe Scaling in the Level 1 BLAS
  ACM Trans Math Softw 44:1--28
  https://doi\&.org/10\&.1145/3061665
.fi
.PP
 
.RE
.PP

.PP
Definition at line \fB91\fP of file \fBdrotg\&.f90\fP\&.
.SH "Author"
.PP 
Generated automatically by Doxygen for LAPACK from the source code\&.