### Copyright (C) 1996 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 2, 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, write to the Free
### Software Foundation, 59 Temple Place - Suite 330, Boston, MA
### 02111-1307, USA.

## usage: poly (x)
##
## If A is a square n-by-n matrix, poly (A) is the row vector of 
## the coefficients of det (z * eye(n) - A), the characteristic
## polynomial of A.
##
## If x is a vector, poly (x) is a vector of coefficients of the
## polynomial whose roots are the elements of x.

function y = poly (x)

  ## Written by KH (Kurt.Hornik@neuro.tuwien.ac.at) Dec 24, 1993.

  if (nargin != 1)
    usage ("poly (x)");
  endif

  m = min (size (x));
  n = max (size (x));
  if (m == 0)
    y = 1;
  elseif (m == 1)
    v = x;
  elseif (m == n)
    v = eig (x);
  else
    usage ("poly (x), where x is a vector or a square matrix");
  endif
  
  y = zeros (1, n+1);
  y(1) = 1;
  for j = 1:n;
    y(2:(j+1)) = y(2:(j+1)) - v(j) .* y(1:j);
  endfor
  
  if (all (all (imag (x) == 0)))
    y = real (y);
  endif
  
endfunction