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+.\" Copyright (c) 1991 The Regents of the University of California.
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed by the University of
+.\"	California, Berkeley and its contributors.
+.\" 4. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"     from: @(#)atan2.3	5.1 (Berkeley) 5/2/91
+.\"	$Id: atan2.3,v 1.3 1993/08/14 13:42:32 mycroft Exp $
+.\"
+.Dd May 2, 1991
+.Dt ATAN2 3
+.Os
+.Sh NAME
+.Nm atan2
+.Nd arc tangent function of two variables
+.Sh SYNOPSIS
+.Fd #include <math.h>
+.Ft double
+.Fn atan2 "double y" "double x"
+.Sh DESCRIPTION
+The
+.Xr atan2
+function computes the principal value of the arc tangent of
+.Ar y/ Ns Ar x ,
+using the signs of both arguments to determine the quadrant of
+the return value.
+.Sh RETURN VALUES
+The
+.Xr atan2
+function, if successful,
+returns the arc tangent of 
+.Ar y/ Ns Ar x
+in the range
+.Bk -words
+.Bq \&- Ns \*(Pi , \&+ Ns \*(Pi
+.Ek
+radians.
+If both
+.Ar x
+and
+.Ar y
+are zero, the global variable
+.Va errno
+is set to
+.Er EDOM .
+On the
+.Tn VAX :
+.Bl -column atan_(y,x)_:=____  sign(y)_(Pi_atan2(Xy_xX))___
+.It Fn atan2 y x No := Ta
+.Fn atan y/x Ta
+if
+.Ar x
+> 0,
+.It Ta sign( Ns Ar y Ns )*(\*(Pi -
+.Fn atan "\\*(Bay/x\\*(Ba" ) Ta
+if
+.Ar x
+< 0,
+.It Ta
+.No 0 Ta
+if x = y = 0, or
+.It Ta
+.Pf sign( Ar y Ns )*\\*(Pi/2 Ta
+if
+.Ar x
+= 0 \*(!=
+.Ar y .
+.El
+.Sh NOTES
+The function
+.Fn atan2
+defines "if x > 0,"
+.Fn atan2 0 0
+= 0 on a
+.Tn VAX
+despite that previously
+.Fn atan2 0 0
+may have generated an error message.
+The reasons for assigning a value to
+.Fn atan2 0 0
+are these:
+.Bl -enum -offset indent
+.It
+Programs that test arguments to avoid computing
+.Fn atan2 0 0
+must be indifferent to its value.
+Programs that require it to be invalid are vulnerable
+to diverse reactions to that invalidity on diverse computer systems.
+.It
+The
+.Fn atan2
+function is used mostly to convert from rectangular (x,y)
+to polar
+.if n\
+(r,theta)
+.if t\
+(r,\(*h)
+coordinates that must satisfy x =
+.if n\
+r\(**cos theta
+.if t\
+r\(**cos\(*h
+and y =
+.if n\
+r\(**sin theta.
+.if t\
+r\(**sin\(*h.
+These equations are satisfied when (x=0,y=0)
+is mapped to
+.if n \
+(r=0,theta=0)
+.if t \
+(r=0,\(*h=0)
+on a VAX.  In general, conversions to polar coordinates
+should be computed thus:
+.Bd -unfilled -offset indent
+.if n \{\
+r	:= hypot(x,y);  ... := sqrt(x\(**x+y\(**y)
+theta	:= atan2(y,x).
+.\}
+.if t \{\
+r	:= hypot(x,y);  ... := \(sr(x\u\s82\s10\d+y\u\s82\s10\d)
+\(*h	:= atan2(y,x).
+.\}
+.Ed
+.It
+The foregoing formulas need not be altered to cope in a
+reasonable way with signed zeros and infinities
+on a machine that conforms to
+.Tn IEEE 754 ;
+the versions of
+.Xr hypot 3
+and
+.Fn atan2
+provided for
+such a machine are designed to handle all cases.
+That is why
+.Fn atan2 \(+-0 \-0
+= \(+-\*(Pi
+for instance.
+In general the formulas above are equivalent to these:
+.Bd -unfilled -offset indent
+.if n \
+r := sqrt(x\(**x+y\(**y); if r = 0 then x := copysign(1,x);
+.if t \
+r := \(sr(x\(**x+y\(**y);\0\0if r = 0 then x := copysign(1,x);
+.Ed
+.El
+.Sh SEE ALSO
+.Xr acos 3 ,
+.Xr asin 3 ,
+.Xr atan 3 ,
+.Xr cos 3 ,
+.Xr cosh 3 ,
+.Xr sin 3 ,
+.Xr sinh 3 ,
+.Xr tan 3 ,
+.Xr tanh 3 ,
+.Xr math 3 ,
+.Sh STANDARDS
+The
+.Fn atan2
+function conforms to
+.St -ansiC .