--- /dev/null
+## Copyright (C) 1993-2012 Dirk Laurie
+##
+## 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} {} invhilb (@var{n})
+## Return the inverse of the Hilbert matrix of order @var{n}. This can be
+## computed exactly using
+## @tex
+## $$\eqalign{
+## A_{ij} &= -1^{i+j} (i+j-1)
+## \left( \matrix{n+i-1 \cr n-j } \right)
+## \left( \matrix{n+j-1 \cr n-i } \right)
+## \left( \matrix{i+j-2 \cr i-2 } \right)^2 \cr
+## &= { p(i)p(j) \over (i+j-1) }
+## }$$
+## where
+## $$
+## p(k) = -1^k \left( \matrix{ k+n-1 \cr k-1 } \right)
+## \left( \matrix{ n \cr k } \right)
+## $$
+## @end tex
+## @ifnottex
+##
+## @example
+## @group
+##
+## (i+j) /n+i-1\ /n+j-1\ /i+j-2\ 2
+## A(i,j) = -1 (i+j-1)( )( ) ( )
+## \ n-j / \ n-i / \ i-2 /
+##
+## = p(i) p(j) / (i+j-1)
+##
+## @end group
+## @end example
+##
+## @noindent
+## where
+##
+## @example
+## @group
+## k /k+n-1\ /n\
+## p(k) = -1 ( ) ( )
+## \ k-1 / \k/
+## @end group
+## @end example
+##
+## @end ifnottex
+## The validity of this formula can easily be checked by expanding
+## the binomial coefficients in both formulas as factorials. It can
+## be derived more directly via the theory of Cauchy matrices.
+## See J. W. Demmel, @cite{Applied Numerical Linear Algebra}, p. 92.
+##
+## Compare this with the numerical calculation of @code{inverse (hilb (n))},
+## which suffers from the ill-conditioning of the Hilbert matrix, and the
+## finite precision of your computer's floating point arithmetic.
+## @seealso{hilb}
+## @end deftypefn
+
+## Author: Dirk Laurie <dlaurie@na-net.ornl.gov>
+
+function retval = invhilb (n)
+
+ if (nargin != 1)
+ print_usage ();
+ elseif (! isscalar (n))
+ error ("invhilb: N must be a scalar integer");
+ endif
+
+ ## The point about the second formula above is that when vectorized,
+ ## p(k) is evaluated for k=1:n which involves O(n) calls to bincoeff
+ ## instead of O(n^2).
+ ##
+ ## We evaluate the expression as (-1)^(i+j)*(p(i)*p(j))/(i+j-1) except
+ ## when p(i)*p(j) would overflow. In cases where p(i)*p(j) is an exact
+ ## machine number, the result is also exact. Otherwise we calculate
+ ## (-1)^(i+j)*p(i)*(p(j)/(i+j-1)).
+ ##
+ ## The Octave bincoeff routine uses transcendental functions (gammaln
+ ## and exp) rather than multiplications, for the sake of speed.
+ ## However, it rounds the answer to the nearest integer, which
+ ## justifies the claim about exactness made above.
+
+ retval = zeros (n);
+ k = [1:n];
+ p = k .* bincoeff (k+n-1, k-1) .* bincoeff (n, k);
+ p(2:2:n) = -p(2:2:n);
+ if (n < 203)
+ for l = 1:n
+ retval(l,:) = (p(l) * p) ./ [l:l+n-1];
+ endfor
+ else
+ for l = 1:n
+ retval(l,:) = p(l) * (p ./ [l:l+n-1]);
+ endfor
+ endif
+
+endfunction
+
+
+%!assert (invhilb (1), 1)
+%!assert (invhilb (2), [4, -6; -6, 12])
+%!test
+%! result4 = [16 , -120 , 240 , -140;
+%! -120, 1200 , -2700, 1680;
+%! 240 , -2700, 6480 , -4200;
+%! -140, 1680 , -4200, 2800];
+%! assert (invhilb (4), result4);
+%!assert (abs (invhilb (7) * hilb (7) - eye (7)) < sqrt (eps))
+
+%!error invhilb ()
+%!error invhilb (1, 2)
+%!error <N must be a scalar integer> invhilb ([1, 2])
+