# Copyright (C) 1995 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, 675 Mass Ave, Cambridge, MA 02139, USA.

function A = compan (c)

# usage: compan (c)
#
# Compute the companion matrix corresponding to polynomial vector c.
#
# In octave a polynomial is represented by it's coefficients (arranged
# in descending order). For example a vector c of length n+1 corresponds
# to the following nth order polynomial
#
#   p(x) = c(1) x^n + ... + c(n) x + c(n+1).
#
# The corresponding companion matrix is
#         _                                                        _
#        |  -c(2)/c(1)   -c(3)/c(1)  ...  -c(n)/c(1)  -c(n+1)/c(1)  |
#        |       1            0      ...       0             0      |
#        |       0            1      ...       0             0      |
#    A = |       .            .   .            .             .      |
#        |       .            .       .        .             .      |
#        |       .            .           .    .             .      |
#        |_      0            0      ...       1             0     _|
#
# The eigenvalues of the companion matrix are equal to the roots of the
# polynomial.
#
# SEE ALSO: poly, roots, residue, conv, deconv, polyval, polyderiv, polyinteg

# Written by Tony Richardson (amr@mpl.ucsd.edu) June 1994.

  if (nargin != 1)
    usage ("compan (vector)");
  endif

  if(is_matrix (c))
    error("compan: expecting a vector argument.");
  endif

# Ensure that c is a row vector.

  if(rows(c) > 1)
    c = c.';
  endif

  n = length (c);
  A = diag (ones (n-2, 1), -1);
  A (1, :) = -c (2:n) /c (1);

endfunction