/usr/share/calc/help/agd is in apcalc-common 2.12.5.0-1.
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 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83  | NAME
    agd - inverse gudermannian function
SYNOPSIS
    agd(z [,eps])
TYPES
    z		number (real or complex)
    eps		nonzero real, defaults to epsilon()
    return	number or infinite error value
DESCRIPTION
    Calculate the inverse gudermannian of z to a nultiple of eps with
    errors in real and imaginary parts less in absolute value than .75 * eps,
    or an error value if z is very close to one of the one of the branch
    points of agd(z)..
    agd(z) is usually defined initially for real z with abs(z) < pi/2 by
    one of the formulae
		 agd(z) = ln(sec(z) + tan(z))
			= 2 * atanh(tan(z/2))
			= asinh(tan(z)),
    or as the integral from 0 to z of (1/cos(t))dt.  For complex z, the
    principal branch, approximated by gd(z, eps), has cuts along the real
    axis outside -pi/2 < z < pi/2.
    If z = x + i * y and abs(x) < pi/2, agd(z) is given by
	agd(z) = atanh(sin(x)/cosh(y)) + i * atan(sinh(y)/cos(x)>
EXAMPLE
    ; print agd(1, 1e-5), agd(1, 1e-10), agd(1, 1e-15)
    1.22619 1.2261911709 1.226191170883517
    ; print agd(2, 1e-5), agd(2, 1e-10)
    1.52345-3.14159i 1.5234524436-3.1415926536i
    ; print agd(5, 1e-5), agd(5, 1e-10), agd(5, 1e-15)
    -1.93237 -1.9323667197 -1.932366719745925
    ; print agd(1+2i, 1e-5), agd(1+2i, 1e-10)
    .22751+1.42291i .2275106584+1.4229114625i
LIMITS
    none
LINK LIBRARY
    COMPLEX *c_agd(COMPLEX *x, NUMBER *eps)
SEE ALSO
    gd, exp, ln, sin, sinh, etc.
## 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: agd,v 30.1 2007/03/16 11:10:42 chongo Exp $
## @(#) $Source: /usr/local/src/bin/calc/help/RCS/agd,v $
##
## Under source code control:	1997/09/06 20:03:34
## File existed as early as:	1997
##
## chongo <was here> /\oo/\	http://www.isthe.com/chongo/
## Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/
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