function A = companc(p)
%COMPAN  COMPANC(P), where P is an (n+1)-vector, is the n-by-n companion matrix
%        whose first row is -P(2:n+1)/P(1).
%        Special case: if P is a scalar then COMPAN(P) is the P-by-P matrix
%                      COMPAN(1:P+1).

%  References:
%  J.H. Wilkinson, The Algebraic Eigenvalue Problem, Oxford University Press,
%       1965, p. 12.
%  C. Kenney and A.J. Laub, Controllability and stability radii for companion
%       form systems, Math. Control Signals Systems, 1 (1988), pp. 239-256.
%       (Gives explicit formulas for the singular values of COMPAN(P).)

n = max(size(p));

%  Handle scalar p.
if n == 1
   n = p+1;
   p = 1:n;
end

p = p(:)';                    % Ensure p is a row vector.

% Construct matrix of order n-1.
if n == 2
   A = 1;
else
    A = diag(ones(1,n-2),-1);
    A(1,:) = -p(2:n)/p(1);
end