% -*- texinfo -*-
% @deftypefn {Function File} {} compan (@var{c})
% Compute the companion matrix corresponding to polynomial coefficient
% vector @var{c}.
%
% The companion matrix is
% @tex
% $$
% A = \left[\matrix{
% -c_2/c_1 & -c_3/c_1 & \cdots & -c_N/c_1 & -c_{N+1}/c_1\cr
% 1 & 0 & \cdots & 0 & 0 \cr
% 0 & 1 & \cdots & 0 & 0 \cr
% \vdots & \vdots & \ddots & \vdots & \vdots \cr
% 0 & 0 & \cdots & 1 & 0}\right].
% $$
% @end tex
% @ifnottex
%
% @c Set example in small font to prevent overfull line
% @smallexample
% _ _
% | -c(2)/c(1) -c(3)/c(1) @dots{} -c(N)/c(1) -c(N+1)/c(1) |
% | 1 0 @dots{} 0 0 |
% | 0 1 @dots{} 0 0 |
% A = | . . . . . |
% | . . . . . |
% | . . . . . |
% |_ 0 0 @dots{} 1 0 _|
% @end smallexample
% @end ifnottex
%
% The eigenvalues of the companion matrix are equal to the roots of the
% polynomial.
% @seealso{poly, roots, residue, conv, deconv, polyval, polyderiv,
% polyinteg}
% @end deftypefn