/usr/share/calc/help/inverse 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.
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    inverse - inverse of value
SYNOPSIS
    inverse(x)
TYPES
    If x is an object of type xx, the function xx_inv has to have
    been defined; any conditions on x and the nature of the returned
    value will depend on the definition of xx_inv.
    For non-object x:
    x		nonzero number (real or complex) or nonsingular matrix
    return	number or matrix
DESCRIPTION
    For real or complex x, inverse(x) returns the value of 1/x.
    If x is a nonsingular n x n matrix and its elements are numbers or
    objects for which the required arithmetic operations are defined,
    inverse(x) returns the matrix m for which m * x = x * m = the unit
    n x n matrix.  The inverse m will have the same index limits as x.
EXAMPLE
    ; print inverse(5/4), inverse(-2/7), inverse(3 + 4i)
    .8 -3.5 .12-.16i
    ; mat A[2,2] = {2,3,5,7}
    ; print inverse(A)
    mat [2,2] (4 elements, 4 nonzero):
	[0,0] = -7
	[0,1] = 3
	[1,0] = 5
	[1,1] = -2
LIMITS
    none
LINK LIBRARY
    void invertvalue(VALUE *x, VALUE *vres)
    NUMBER *qinv(NUMBER *x)
    COMPLEX *c_inv(COMPLEX *x)
    MATRIX *matinv(MATRIX *x)
SEE ALSO
## 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: inverse,v 30.1 2007/03/16 11:10:42 chongo Exp $
## @(#) $Source: /usr/local/src/cmd/calc/help/RCS/inverse,v $
##
## Under source code control:	1996/03/12 23:10:01
## File existed as early as:	1996
##
## chongo <was here> /\oo/\	http://www.isthe.com/chongo/
## Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/
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