# NAG FL Interfacef06bcf (dcsg)

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## 1Purpose

f06bcf reconstructs the parameters $c$ and $s$ of a real plane rotation from the tangent of that rotation.

## 2Specification

Fortran Interface
 Subroutine f06bcf ( t, c, s)
 Real (Kind=nag_wp), Intent (In) :: t Real (Kind=nag_wp), Intent (Out) :: c, s
#include <nag.h>
 void f06bcf_ (const double *t, double *c, double *s)
The routine may be called by the names f06bcf or nagf_blas_dcsg.

## 3Description

f06bcf reconstructs the parameters $c$ and $s$ of a real plane rotation from the value of the tangent $t$, as returned by f06baf:
 $c=11+t2 , s=ct,$
so that $c\ge 0$ and $s$ has the same sign as $t$.
If $|t|<\sqrt{\epsilon }$, where $\epsilon$ is the machine precision, the routine sets $c=1$ and $s=t$; if $|t|>1/\sqrt{\epsilon }$, the routine sets $c=\frac{1}{|t|}$ and $s=\mathrm{sign}t$.

None.

## 5Arguments

1: $\mathbf{t}$Real (Kind=nag_wp) Input
On entry: the value $t$, the tangent of the rotation.
2: $\mathbf{c}$Real (Kind=nag_wp) Output
On exit: the value $c$, the cosine of the rotation.
3: $\mathbf{s}$Real (Kind=nag_wp) Output
On exit: the value $s$, the sine of the rotation.

None.

Not applicable.

## 8Parallelism and Performance

f06bcf is not threaded in any implementation.