/usr/share/calc/help/atan2 is in apcalc-common 2.12.4.4-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 | NAME
    atan2 - angle to point
SYNOPSIS
    atan2(y, x, [,eps])
TYPES
    y		real
    x		real
    eps		nonzero real, defaults to epsilon()
    return	real
DESCRIPTION
    If x and y are not both zero, atan2(y, x, eps) returns, as a multiple of
    eps with error less than abs(eps), the angle t such that
    -pi < t <= pi and x = r * cos(t), y = r * sin(t), where
    r > 0.  Usually the error does not exceed abs(eps)/2.
    Note that by convention, y is the first argument; if x > 0,
    atan2(y, x) = atan(y/x).
    To conform to the 4.3BSD ANSI/IEEE 754-1985 math lib, atan2(0,0)
    returns 0.
EXAMPLE
    ; print atan2(0,0), atan2(1,sqrt(3)), atan2(17,53,1e-100)
    0 ~.52359877559829887307 ~.31038740713235146535
LIMITS
    none
LINK LIBRARY
    NUMBER *qatan2(NUMBER *y, *x, *acc)
SEE ALSO
    acos, asin, atan, cos, epsilon, sin, tan
## Copyright (C) 1999  Landon Curt Noll
##
## Calc is open software; you can redistribute it and/or modify it under
## the terms of the version 2.1 of the GNU Lesser General Public License
## as published by the Free Software Foundation.
##
## Calc is distributed in the hope that it will be useful, but WITHOUT
## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
## or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU Lesser General
## Public License for more details.
##
## A copy of version 2.1 of the GNU Lesser General Public License is
## distributed with calc under the filename COPYING-LGPL.  You should have
## received a copy with calc; if not, write to Free Software Foundation, Inc.
## 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
##
## @(#) $Revision: 30.1 $
## @(#) $Id: atan2,v 30.1 2007/03/16 11:10:42 chongo Exp $
## @(#) $Source: /usr/local/src/cmd/calc/help/RCS/atan2,v $
##
## Under source code control:	1994/03/19 01:40:26
## File existed as early as:	1994
##
## chongo <was here> /\oo/\	http://www.isthe.com/chongo/
## Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/
 |