## Copyright (C) 1993-2011 John W. Eaton
##
## This file is part of Octave.
##
## Octave is free software; you can redistribute it and/or modify it
## under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 3 of the License, or (at
## your option) any later version.
##
## Octave 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
## General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with Octave; see the file COPYING.  If not, see
## <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {Function File} {} hilb (@var{n})
## Return the Hilbert matrix of order @var{n}.  The @math{i,j} element
## of a Hilbert matrix is defined as
## @tex
## $$
## H (i, j) = {1 \over (i + j - 1)}
## $$
## @end tex
## @ifnottex
##
## @example
## H (i, j) = 1 / (i + j - 1)
## @end example
##
## @end ifnottex
##
## Hilbert matrices are close to being singular which make them difficult to
## invert with numerical routines.
## Comparing the condition number of a random matrix 5x5 matrix with that of
## a Hilbert matrix of order 5 reveals just how difficult the problem is.
##
## @example
## @group
## cond (rand (5))
##     @result{} 14.392
## cond (hilb (5))
##     @result{} 4.7661e+05
## @end group
## @end example
##
## @seealso{invhilb}
## @end deftypefn

## Author: jwe

function retval = hilb (n)


  if (nargin != 1)
    print_usage ();
  endif

  nmax = length (n);
  if (nmax == 1)
    retval = zeros (n);
    tmp = 1:n;
    for i = 1:n
      retval (i, :) = 1.0 ./ (tmp + (i - 1));
    endfor
  else
    error ("hilb: expecting scalar argument, found something else");
  endif

endfunction

%!assert((hilb (2) == [1, 1/2; 1/2, 1/3]
%! && hilb (3) == [1, 1/2, 1/3; 1/2, 1/3, 1/4; 1/3, 1/4, 1/5]));

%!error hilb ();

%!error hilb (1, 2);