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+## 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])
+